ISDN to VOIP Migration Australia: Complete Guide (2026)

ISDN to VOIP migration in Australia follows a well-defined path used by thousands of businesses every year, and the timeline is typically 2 to 6 weeks with zero call downtime when planned correctly. This guide covers every ISDN service type sold in Australia, the exact SIP trunking equivalents that replace them, and the specific migration steps that apply to Australian businesses dealing with Telstra and carrier-managed copper services. If your carrier has told you ISDN is going away but has not shown you a clear alternative, this guide will close that gap. By the end, you will know which replacement fits your setup, what it will cost in AUD, and how to migrate without dropping a single call.

What Is ISDN and Why Is It Being Shut Down?

ISDN stands for Integrated Services Digital Network. It is a set of communication standards that transmit voice and data simultaneously over traditional copper telephone lines using digital signalling. ISDN was a significant advancement over analog PSTN when it was introduced in the 1980s and 1990s, and it became the standard for business telephone systems across Australia for decades. The technology provided reliable, predictable call quality and, crucially, delivered multiple simultaneous call channels over a single physical connection.In Australia, three ISDN service types were sold to business customers. ISDN2 (also called ISDN2e or Basic Rate Interface, BRI) delivers 2 simultaneous call channels over a single copper pair. ISDN10, ISDN20, and ISDN30 (collectively called Primary Rate Interface, PRI) deliver 10, 20, or 30 simultaneous call channels over a single physical circuit. Larger businesses and call centres used ISDN30 to handle high call volumes. Medium businesses used ISDN10 or ISDN20. Small businesses and professional practices used ISDN2.ISDN is being shut down for one reason: the copper network it runs on is being decommissioned. Australia's NBN rollout replaced the PSTN copper network with fibre, fixed wireless, and satellite infrastructure. ISDN cannot run over NBN. Telstra and the other carriers that resell ISDN services have been progressively withdrawing ISDN products since 2022, and the shutdown is expected to be complete in the near term. New ISDN connections have not been available for several years. Existing connections are being terminated on a carrier-by-carrier timeline. See the PSTN shutdown Australia guide for the full copper decommission timeline.If you are currently running ISDN services, this is not a crisis. It is actually an opportunity. The replacement technology, SIP trunking, delivers the same channel-based call capacity over your existing internet connection, costs less per month, and adds features your ISDN service never had. The migration path is well understood and thousands of Australian businesses have already completed it.

Understanding Your Current ISDN Setup

Before planning your migration, you need a clear picture of exactly what ISDN services your business is running. Start by identifying which service type you have and how many channels you are paying for. This information appears on your phone bill under the service description, or your carrier account manager can confirm it.

ISDN2 (Basic Rate Interface)

ISDN2 provides 2 bearer channels (B-channels) and 1 data channel (D-channel) over a single copper pair, commonly described as 2B+D. Each B-channel carries one call simultaneously, so ISDN2 gives you 2 concurrent calls on one number. ISDN2 was sold to small businesses, medical practices, trades companies, and professional services firms that needed more than a single analog line but did not require the capacity of a PRI service. Many ISDN2 customers had their existing PBX connected directly to the ISDN2 line.

ISDN10 and ISDN20 (Primary Rate Interface)

ISDN PRI services deliver 23 or 30 B-channels over a single physical circuit, configured in blocks sold as ISDN10, ISDN20, or ISDN30 depending on the number of channels actually activated. An ISDN10 service uses 10 of the available channels. ISDN20 uses 20. This is the service type used by medium-sized businesses with in-house PBX systems, where the PRI circuit connects directly to the PBX and the system manages call routing internally. If your business has a Panasonic, NEC, Avaya, or similar PBX with a PRI card installed, you are almost certainly running ISDN PRI.

ISDN30 (High-Capacity PRI)

ISDN30 is the full 30-channel configuration of PRI, used by larger businesses, call centres, and multi-site operations where simultaneous call volumes are high. If you are running ISDN30, your migration will typically be more complex because the call capacity is business-critical and the replacement SIP trunk configuration needs to be tested thoroughly before cutover. That said, SIP trunking at this scale is straightforward for experienced providers and the migration process is well established.

Other ISDN-Connected Equipment to Audit

Before you plan any migration, audit every device in your building that connects to an ISDN or PSTN line. Many businesses discover during this audit that they have lines they had forgotten about. Check for: EFTPOS terminals (some older models use PSTN/ISDN dial-up for transaction processing), fax machines (many businesses still receive faxes on a dedicated ISDN line), alarm systems and monitored security (alarms often dial through a PSTN or ISDN line to the monitoring centre), lift intercoms and emergency phones, and any modem-based connections used for backup or point-of-sale systems. Each of these will need a separate migration plan. The section below covers fax, EFTPOS, and alarm handling in detail.

SIP Trunking: The Direct ISDN Replacement

SIP trunking is the direct functional replacement for ISDN. Where ISDN delivers call channels over copper, SIP trunking delivers call channels over your internet connection using the SIP (Session Initiation Protocol) standard. The key concept to understand is the channel-to-channel mapping: one ISDN channel equals one SIP channel, and one SIP channel equals one simultaneous call. This makes capacity planning straightforward.SIP trunks connect to the same type of PBX that ISDN connects to. If you have an on-premise PBX with a PRI card, the migration path is to replace the ISDN PRI circuit with a SIP trunk at the same channel count, configure the PBX to use SIP instead of ISDN signalling, and port your numbers from the ISDN service to the new SIP provider. Your phones, extension numbering, call flows, and staff experience remain unchanged. The PBX still routes calls the same way. The only change is what comes into the back of the PBX.

Mapping ISDN Services to SIP Trunk Equivalents

The following table maps each ISDN service type to its SIP trunking equivalent and provides indicative monthly cost comparisons in AUD. Actual SIP trunk pricing varies by provider, but the direction is consistent: SIP is cheaper than ISDN for equivalent capacity. The cost savings on a like-for-like replacement are significant, typically 40 to 60 percent of the current ISDN monthly spend. On an ISDN30 service costing $900 per month, migration to an equivalent SIP trunk arrangement can save $500 to $600 per month. That saving alone justifies the migration project many times over.Beyond direct cost comparison, SIP trunking delivers capabilities ISDN cannot match: geographic number portability (your number is no longer tied to a physical location), disaster recovery routing (calls can automatically divert to a backup destination if your internet fails), real-time call analytics, and instant capacity scaling without waiting for a technician. If your business grows and needs more concurrent channels, a SIP trunk scales immediately via your provider's portal. ISDN capacity changes required physical circuit upgrades and multi-week lead times.

ISDN2 Replacement: What Small Businesses Need to Know

If you are on ISDN2, you have two clean replacement options. The first option is SIP trunking connected to your existing PBX. If your PBX supports SIP (most PBX systems sold in Australia since 2005 do, or can with a firmware update), you can replace the ISDN2 line with a 2-channel SIP trunk and maintain your existing setup. The second option is to take the migration as an opportunity to move to a hosted PBX, which removes the on-premise PBX hardware entirely and shifts all call management to the cloud.For small businesses with a PBX that is already ageing, the hosted PBX path often makes more sense. A hosted PBX from a provider like Maxotel costs roughly $30 to $60 AUD per user per month and includes unlimited local and national calls, a cloud-based phone management portal, voicemail to email, call recording, auto-attendant, and mobile app access. There is no PBX hardware to maintain, no capital expenditure, and support is included. If your ISDN2 service is connected to a PBX that is more than 5 to 7 years old, replacing both simultaneously is usually the lower-cost and lower-risk option. See the hosted PBX vs on-premise comparison guide for a full breakdown.The one case where SIP trunking to an existing ISDN2-connected PBX makes sense for a small business is when the PBX is relatively new, fully featured, and staff are trained on it. In that case, swapping only the ISDN2 line for a SIP trunk minimises disruption and preserves the existing investment.

ISDN10 and ISDN30 Replacement: Mid-Size and Larger Businesses

Businesses running ISDN10, ISDN20, or ISDN30 almost always have an on-premise PBX handling internal call routing. The migration path in this case is SIP trunking as a direct replacement for the ISDN PRI circuit. The PBX remains in place. The change happens at the connection point between the PBX and the outside world.Technically, this means configuring a SIP trunk on the PBX (either natively if the PBX supports SIP, or via a SIP-to-ISDN gateway if the PBX only speaks ISDN). Most modern PBX systems from Panasonic, NEC, Cisco, Avaya, and similar vendors support SIP trunks natively. Older PBX hardware may require a gateway device, typically called a VoIP gateway or ISDN gateway, that sits between the PBX and the SIP trunk and handles protocol translation. Your SIP provider or a qualified VOIP engineer can advise on whether your specific PBX requires a gateway.One consideration for larger ISDN migrations is internet connection capacity. An ISDN30 replacement requires 30 concurrent call channels. At 100 kbps per channel, that is 3 Mbps of upload bandwidth dedicated to calls. Most Australian NBN business connections have more than adequate capacity, but it is worth confirming before migration. Use the VoIP Bandwidth Calculator to run the numbers for your specific situation. If your connection is NBN 50 or higher with a business-grade service level agreement, you are almost certainly fine.

Hosted PBX as an Alternative Migration Path

Not every ISDN migration has to preserve the on-premise PBX. For many businesses, the ISDN decommission is a trigger to evaluate whether the on-premise PBX itself is still the right architecture. Cloud-hosted PBX has matured significantly and now delivers everything an on-premise PBX can, without the hardware maintenance, capital cost, or on-site support requirements. For an introduction to what a hosted PBX actually does, see the best VOIP phone system guide for Australian small businesses.If your on-premise PBX is approaching end of life, if your maintenance costs are rising, or if you have staff working remotely who need to be part of the phone system, a hosted PBX migration may be more cost-effective than a like-for-like SIP trunk replacement. The hosted path requires replacing desk phones with SIP-compatible handsets (if your existing phones are not SIP-capable) and migrating your number configuration to the cloud platform. The upside is that you eliminate the on-premise hardware, reduce IT overhead, and gain mobile app integration and remote extension capability. Read the hosted PBX vs on-premise comparison to see which architecture fits your current situation.The hosted path is not always the right choice. If you have a large, complex on-premise PBX with extensive customisation, integration with a CRM or contact centre system, or a specialised call flow that your IT team has tuned over years, preserving that investment with a SIP trunk swap is often more pragmatic. The migration decision should be made based on total cost of ownership, not just the immediate line cost.

Handling Fax, EFTPOS, and Alarm Lines During Migration

This is the section most ISDN migration guides skip, and it is the section that most commonly causes post-migration problems. Fax machines, EFTPOS terminals, alarm systems, and lift intercoms that connect via ISDN or PSTN lines do not migrate automatically. Each one needs to be addressed before you disconnect the old service.

Fax Lines

Traditional fax machines transmit over analog signalling and are fundamentally incompatible with pure SIP trunking. There are three migration options for fax. Option one is online fax services (T.38 fax over IP). Providers like FaxBetter, eFax, and some VOIP providers support T.38, which is the standard for transmitting fax over IP networks. This requires either a T.38-capable fax machine or an ATA (analog telephone adapter) that converts your existing fax machine to work over IP. Option two is a dedicated analog line using an ATA. An ATA plugged into your internet connection gives your fax machine an analog-style connection it can use normally, while the actual transmission happens over VOIP. Option three is to simply retire the fax machine and migrate to email-based document exchange. If your business still needs to receive occasional faxes from third parties (health departments, legal firms, suppliers), an online fax number that delivers to email costs around $10 to $15 AUD per month and eliminates the hardware entirely.

EFTPOS Terminals

Most EFTPOS terminals sold in Australia since 2015 use 4G mobile connectivity or direct ethernet connection for transaction processing and do not rely on PSTN or ISDN lines. If you have a modern EFTPOS terminal from ANZ, Commonwealth Bank, Tyro, or similar, it is almost certainly already running on mobile data and is unaffected by your ISDN migration. However, if you are running older EFTPOS equipment, check with your payment provider before migrating. Some older dial-up terminals used an ISDN or PSTN line, and these will stop working when the line is disconnected. Your bank's merchant services team can confirm which connectivity method your terminal uses and arrange an equipment upgrade if needed.

Alarm Systems and Security Monitoring

Alarm systems that dial through to a monitoring centre via PSTN or ISDN need specific attention. The alarm communicator in your panel dials a phone number when triggered. If that communicator is programmed for an analog dial tone, it will not work over a SIP trunk without a compatible ATA. The safest approach is to contact your alarm monitoring company before migration and ask them to assess the communicator type and recommend a solution. Most alarm companies have dealt with ISDN migration before and can supply a 4G communicator upgrade that removes the dependence on fixed-line connectivity entirely. This is also a more resilient option, since 4G continues working even if your internet connection fails.

The ISDN to VOIP Migration Process: Step by Step

This is the complete migration process for an Australian business moving from ISDN to VOIP. The timeline varies based on number porting requirements and the complexity of your existing setup, but for most businesses the end-to-end process takes 2 to 6 weeks.

Step 1: Audit Your Current Setup (Week 1)

List every service that uses your ISDN connection. Include: main business numbers, direct inward dialling (DID) numbers, fax lines, EFTPOS lines, alarm lines, and lift intercoms. Note which are on the ISDN circuit and which are on separate analog lines. Identify your current carrier, your ISDN service type (BRI or PRI), the channel count, and whether your contract has a minimum term remaining. Minimum terms on ISDN services can result in early termination fees if you cancel before the term expires. Factor this into your migration timing.

Step 2: Choose Your Replacement Architecture (Week 1)

Decide whether you are migrating to SIP trunking with your existing PBX, or moving to a hosted PBX. Use the Phone System Sizing Wizard to model your requirements based on staff count, call volume, and required features. Use the VoIP Cost Calculator to compare the monthly cost of each option against your current ISDN spend. If you have complex requirements or a large team, get a consultation from a specialist VOIP provider before committing to an architecture.

Step 3: Select a SIP Provider and Confirm Technical Compatibility (Week 1 to 2)

Not all VOIP providers are equal in terms of SIP trunk support and PBX compatibility. When evaluating providers, confirm: that they support the SIP trunk channel count you need, that they support T.38 if you are migrating a fax line, that they can port your ISDN numbers (this is a separate porting process from NBN number porting), and that they have experience migrating businesses from your specific PBX make and model. Ask for references from existing customers who have migrated from ISDN. A good provider will have done this many times and will be able to tell you exactly what is involved for your setup.

Step 4: Prepare Your Internet Connection

Confirm that your internet connection has sufficient upload bandwidth for the number of concurrent SIP channels you are migrating to. Also confirm that QoS (Quality of Service) settings are configured on your router to prioritise voice traffic. SIP calls are sensitive to jitter and packet loss. If your internet connection is shared with data-intensive business applications (large file transfers, video conferencing, cloud backups), voice calls can be degraded if voice traffic is not prioritised. Most business-grade routers support QoS configuration. Your VOIP provider can advise on recommended settings.

Step 5: Initiate Number Porting (Week 2 to 3)

Number porting from ISDN to VOIP follows the standard Australian ACMA porting process but has some ISDN-specific considerations. ISDN numbers are classified as geographic numbers under the same rules as standard landlines. Porting an ISDN number to a new provider typically takes 5 to 10 business days for a straightforward residential or small business number. For ISDN PRI numbers with a large DID range, the port may take longer and requires coordination between your current carrier and the new provider. Your new VOIP provider manages the porting process. You do not contact your current carrier to cancel until the port is confirmed complete. Cancelling early will result in the number being permanently lost.During the porting period, your ISDN service remains fully active. You will not experience any call interruption. The actual cutover to the new service happens at a scheduled moment, usually at a time you choose during business hours so you can verify the cut is working before leaving for the day. Read the number porting Australia guide for a full breakdown of the porting process and what to watch for.

Step 6: Configure and Test in Parallel (Week 3 to 4)

Do not do a cold cutover. The best practice is to run your new SIP configuration in parallel with the existing ISDN service for a testing period before porting your numbers. Your VOIP provider can assign temporary test numbers to your new SIP trunk or hosted PBX so you can make and receive calls, test call quality, test call flows and extensions, and verify fax handling, all while your ISDN service continues handling production calls. This parallel running period is your safety net. Any configuration issue discovered during parallel testing can be resolved before it becomes a problem on your main business number.

Step 7: Cutover and Cancel ISDN (Week 4 to 6)

Once the number port is confirmed complete, test incoming calls on your main number to verify they are routing correctly through the new system. Make a test outbound call and verify caller ID is displaying correctly. Check that any IVR prompts, ring group routing, and voicemail are functioning as configured. Once satisfied, you can notify your ISDN carrier to cancel the ISDN service. Keep a record of the cancellation date and confirmation reference. Some carriers continue billing ISDN services after cancellation requests if the instruction is not properly recorded.

Internet Connection Requirements for SIP Trunking

Australia's NBN infrastructure is more than capable of supporting SIP trunking for most business use cases. The key metric is upload bandwidth, not download. Each concurrent SIP call uses approximately 80 to 100 kbps of upload bandwidth when using the G.711 codec (the standard high-quality codec). Using G.729 compression reduces this to approximately 25 to 30 kbps per call, at a minor reduction in audio quality.For an ISDN2 replacement (2 concurrent calls), any NBN service with 5 Mbps upload or better will handle call traffic comfortably alongside normal business internet use. For an ISDN10 replacement (10 concurrent calls), an NBN Business 50 or NBN Business 100 service with guaranteed upload speeds is recommended. For ISDN30 replacement (30 concurrent calls), a business fibre service with 50 Mbps or better upload and a guaranteed SLA is required. Fixed wireless and satellite NBN connections are generally unsuitable for high-channel SIP trunk configurations due to latency variability. Use the VoIP Bandwidth Calculator to model your specific requirements.

Cost Comparison: ISDN Monthly vs SIP Trunk Monthly

The financial case for ISDN migration is straightforward. Australian ISDN services have always been priced at a premium that reflects the infrastructure cost of maintaining copper circuits. SIP trunking runs over internet infrastructure that you are already paying for, and the incremental cost of adding voice channels is much lower.A typical ISDN2 service from an Australian carrier costs $80 to $120 per month for the line rental alone, before call charges. An equivalent 2-channel SIP trunk costs $30 to $50 per month including a local and national call bundle. Annual saving: $600 to $840 per year on a single ISDN2 line. A 10-channel ISDN PRI service typically costs $300 to $500 per month for the circuit. An equivalent 10-channel SIP trunk with a call bundle costs $80 to $150 per month. Annual saving: $1,800 to $4,200 per year. On a 30-channel ISDN30 service, current carrier pricing is often $700 to $1,200 per month. The SIP equivalent is $200 to $350 per month. Annual saving: $4,200 to $10,200 per year.These savings apply before accounting for the reduction in call costs on a per-minute basis, which are also generally lower with modern SIP providers than with legacy ISDN carrier rates. Use the VoIP Cost Calculator for a personalised estimate based on your actual call volume and team size.

Australian Reality: What ISDN Customers Should Know

The PSTN copper network shutdown in Australia is not a future event. It is complete. See the PSTN shutdown Australia guide for the full timeline and what it means for each connection type. If your ISDN service is still active, your carrier is maintaining it on a transition basis, but that arrangement has an end date. Some carriers have already issued specific shutdown notices to ISDN customers with 6 to 12 months lead time. Others have not yet sent formal notices but the service is end-of-life.Number porting from ISDN follows ACMA rules under the Telecommunications Numbering Plan. ISDN numbers are geographic numbers and can be ported to any registered VOIP carrier. The ACMA does not permit carriers to unreasonably delay or obstruct a porting request. If your current ISDN carrier is slow to cooperate with a port, your new provider can escalate through the standard porting dispute process. See the number porting Australia guide for the full porting rights framework.One ISDN-specific porting consideration: if you have a large DID (direct inward dialling) number range attached to your PRI service, the port must cover the entire DDI range as a block. Your new provider needs to support block DDI porting, not just single-number porting. Confirm this capability explicitly before signing up.000 emergency calling is supported on SIP trunking in Australia, but with caveats. Unlike a fixed ISDN line with a registered physical address, SIP trunks are location-independent. When emergency services receive a 000 call from a SIP trunk, they may not automatically know your physical location. Ensure your VOIP provider has correctly registered your site address in the Emergency Call Person Register and confirm the process for multi-site or remote work setups. Most Australian business VOIP providers handle this as part of onboarding, but it is worth verifying.Power outage is a consideration for hosted PBX and SIP trunking that did not apply to ISDN. ISDN lines carried their own power from the exchange. Your NBN connection and your on-premise equipment rely on mains power. If power is lost, calls will not function without a UPS (uninterruptible power supply) on your modem and networking equipment. For business-critical inbound call handling, configure an overflow destination in your VOIP provider's portal so that calls divert to a mobile number automatically if your internet connection is unreachable.

Migrating from Landline and ISDN Simultaneously

Many businesses have a mix of ISDN services and standard analog PSTN or NBN landlines. If you are in this situation, the most efficient approach is to migrate everything to the same VOIP platform at the same time. Running a separate SIP trunk for the ISDN replacement while also migrating your analog lines means two porting processes, two providers to manage, and two billing relationships. A single VOIP provider can handle both the ISDN port and the analog line port under one contract. Read the migrate landline to VOIP Australia guide for the full process covering the analog side of a combined migration.

Common Mistakes: What Most Businesses Get Wrong with ISDN Migration

These are the three mistakes that consistently cause problems or cost extra money on ISDN migrations in Australia.

Mistake 1: Cancelling the ISDN Service Before the Port Is Complete

This is the most damaging and most common mistake. Businesses sometimes contact their carrier to cancel the ISDN service at the same time as initiating a number port to a new provider. If the carrier processes the cancellation before the port completes, the number is permanently released and cannot be ported. It is gone. You cannot get it back. The correct sequence is: initiate port with new provider, wait for port completion confirmation, then cancel the old service. Never cancel first. Even a few hours matter. The porting rules exist precisely to protect customers from this scenario, but carrier billing systems do not always check for pending ports before processing a cancellation.

Mistake 2: Migrating Voice But Not Auditing Fax, EFTPOS, and Alarm Lines

Businesses focus on the main phone lines during migration planning and discover after cutover that the fax machine is silent, the alarm is not checking in with the monitoring centre, or the EFTPOS terminal is declining transactions. These are not difficult problems to solve, but they need to be identified and addressed before cutover, not after. Complete a thorough audit of every device connected to your ISDN circuit before you begin the migration process. The section above covers the specific handling for each device type.

Mistake 3: Under-Specifying the SIP Trunk Channel Count

Some businesses see ISDN migration as an opportunity to reduce costs and specify fewer SIP channels than they currently have on ISDN, without actually analysing their call patterns first. If you currently have ISDN10 and are running 7 to 9 concurrent calls during peak periods, migrating to a 5-channel SIP trunk to save money will result in callers receiving busy signals. Before reducing channel count, ask your ISDN carrier for a concurrent call report that shows your actual peak channel usage. Most carriers can provide this. Use that data to right-size your SIP trunk. Equally, if the data shows you rarely exceed 4 concurrent calls, there is a legitimate case for reducing from ISDN10 to a 4-channel SIP trunk. Just base the decision on evidence, not assumption.

Your Next Steps: ISDN Migration Checklist

Work through this checklist in order. Each step builds on the previous one and prevents the common mistakes outlined above.Step 1: Identify your ISDN service type (BRI/ISDN2 or PRI/ISDN10/20/30) and channel count. Find this on your current invoice or call your carrier. Step 2: Audit every device connected to your ISDN circuit. List main numbers, fax lines, alarm lines, EFTPOS terminals, and lift intercoms. Decide the migration path for each. Step 3: Check your ISDN contract for minimum terms and early termination fees. Factor this into your migration timing. Step 4: Assess your internet connection upload bandwidth. Use the VoIP Bandwidth Calculator to confirm you have sufficient capacity for your target SIP channel count. Step 5: Decide your replacement architecture: SIP trunking to existing PBX, or hosted PBX. Use the Phone System Sizing Wizard to model requirements. Step 6: Get quotes from at least two VOIP providers. Confirm SIP trunk support, T.38 fax support if needed, and experience migrating from your PBX make and model. Step 7: Sign up with your chosen provider and set up parallel running on temporary numbers. Test call quality, call flows, and all features before any porting. Step 8: Initiate number porting for all numbers you are keeping. Do NOT cancel your ISDN service until all ports are confirmed complete. Step 9: On cutover day, test incoming and outgoing calls on all ported numbers. Verify IVR, ring groups, and voicemail. Step 10: Once confirmed working, cancel your ISDN service with your current carrier. Keep the cancellation confirmation reference.Not sure which path is right for your business? Use the Get a Recommendation form and a specialist will review your setup and provide a personalised migration recommendation at no cost.

Can I keep my existing ISDN phone numbers when I migrate to VOIP?

Yes. ISDN numbers are geographic numbers under the ACMA Telecommunications Numbering Plan and can be ported to any registered VOIP carrier in Australia. The porting process takes 5 to 10 business days for standard numbers. For ISDN PRI services with a large DDI number range, the port covers the entire block and may take slightly longer. Your new VOIP provider manages the porting process. You do not need to contact your current carrier to initiate the port.

How long does an ISDN to VOIP migration take?

Most Australian ISDN migrations complete in 2 to 6 weeks end-to-end. The audit and provider selection phase takes approximately one week. Number porting takes 5 to 10 business days. Parallel running and testing adds another 1 to 2 weeks before cutover. The actual cutover moment, when your main number switches to the new service, takes minutes. Larger or more complex migrations with many numbers or specialised equipment (fax, alarms, EFTPOS) may take longer.

What happens to my fax machine when I migrate from ISDN?

Traditional fax machines use analog signalling and cannot connect directly to a SIP trunk. You have three options: use a T.38 fax over IP service (supported by some VOIP providers), use an ATA (analog telephone adapter) to give your fax machine an analog-style connection over your internet service, or migrate to online fax service that delivers faxes as PDF attachments to email. Online fax services typically cost $10 to $15 AUD per month and are the simplest option for businesses that receive occasional faxes but no longer send them frequently.

Do I need to replace my PBX when migrating from ISDN?

Not necessarily. If your on-premise PBX supports SIP trunking (most systems sold in Australia since 2005 do), you can keep the PBX and simply replace the ISDN line with a SIP trunk. Older PBX hardware that only supports ISDN signalling may require a SIP-to-ISDN gateway device, which sits between the PBX and the SIP trunk and handles protocol translation. The gateway approach extends the useful life of your existing PBX without requiring replacement. However, if your PBX is ageing and approaching end of support, the ISDN migration is a logical time to also evaluate a hosted PBX replacement.

How many SIP channels do I need to replace my ISDN service?

Start with a like-for-like replacement: one SIP channel for every ISDN channel you currently have. Then review your actual peak concurrent call usage before finalising the count. If your carrier can provide a concurrent call report, use it. If you regularly use 7 of your 10 ISDN channels during peak periods, specify at least 8 to 10 SIP channels. If your actual peak is only 4 channels, you may be able to migrate to a 5 or 6 channel SIP trunk and save money on line rental. Use the Phone System Sizing Wizard to model your specific requirements.

Will my alarm system work after ISDN migration?

It depends on the communicator type installed in your alarm panel. If the communicator dials out via PSTN or ISDN, it will not work on a SIP trunk without a compatible ATA or upgrade. The safest approach is to contact your alarm monitoring company before starting your migration. Most will recommend a 4G communicator upgrade that removes the dependence on fixed-line connectivity entirely. This also makes the alarm more resilient, since it continues working even if your internet connection is down.

What is the difference between SIP trunking and hosted PBX for an ISDN replacement?

SIP trunking replaces the ISDN circuit while keeping your existing on-premise PBX. The PBX still handles all internal call routing. SIP trunking is the lower-disruption option when you have a functioning PBX and do not want to change how staff use the phone system. Hosted PBX replaces both the ISDN service and the on-premise PBX with a cloud-based phone management platform. It requires SIP-capable desk phones or softphones but eliminates PBX hardware, reduces IT overhead, and adds remote work capability. Hosted PBX is the better choice if your current PBX is ageing, if you have remote staff, or if your IT team wants to reduce on-site infrastructure.

Can I migrate from ISDN to VOIP without any downtime?

Yes. A properly managed ISDN migration has zero call downtime. The process involves setting up the new VOIP service on temporary numbers, running it in parallel with the active ISDN service, testing thoroughly, and then porting your numbers to the new service. The port cutover happens at a scheduled time. During the porting window, there is a brief period of minutes when calls may not connect, but experienced providers schedule this during low-traffic periods and it is typically imperceptible. Your ISDN service remains fully active until the port is confirmed complete.

Related reading:

• PSTN shutdown timeline
• landline to VoIP migration
• number porting