ATA Adapter for Analogue Phones: Connect Your Existing Handsets to VOIP

This guide covers ATA adapters -- what they are, how they work with Australian NBN-connected VOIP services, which models are commonly available in Australia, and when they make sense for your business. Need to Know Comms is an independent Australian publishing project with direct experience in VOIP deployment across NBN connection types and ISP infrastructure. If you've ever plugged a phone into the green port on your NBN modem and wondered why you can't connect a new Yealink or Grandstream handset to the same service, this guide is exactly what you need to read. You're not behind. The industry made this confusing on purpose.

What Is an ATA Adapter?

An Analog Telephone Adapter (ATA) is a small device that converts the analogue audio signal from a traditional telephone into digital SIP (Session Initiation Protocol) packets that can travel over an IP network. In plain English: it lets an old phone plug into a modern VOIP service.

Analogue phones use a physical audio connection (the RJ11 phone jack, also called an FXS port). VOIP services use SIP protocol over an internet connection. An ATA sits in between, bridging the two worlds. You connect the ATA to your router via Ethernet, plug the analogue phone into the ATA's phone port, and configure the ATA with SIP credentials from your VOIP provider.

ATAs have been around since the early 2000s and have been critical infrastructure for VOIP migration worldwide. In the Australian context, they matter for two distinct reasons: migrating existing office handsets to a new VOIP service, and the widespread but poorly understood ISP ATA situation that affects hundreds of thousands of Australian businesses on NBN.

The Two Very Different ATA Situations in Australia

Situation 1: The ISP-Supplied ATA (The Green Port Trap)

When NBN was rolled out across Australia, ISPs needed a way to deliver phone service to customers who had been on copper landlines. The solution was to include a built-in ATA in the NBN modem/router supplied to customers. That ATA is represented by the green phone port on the back of your modem. Plug any analogue phone into that green port and it rings and dials. Most users assume this is just "how phones work on NBN".

What's actually happening: the ISP configured the built-in ATA with their own VOIP credentials. The ISP controls the SIP server, the SIP username, the SIP password, and the phone number. You don't have access to any of this. You cannot take those credentials and use them with a different VOIP provider. You cannot configure a SIP desk phone (Yealink, Grandstream, Polycom) to register with the ISP's VOIP service -- the ISP has locked this out. You are renting the phone service from the ISP, and the ISP's ATA is the gatekeeper.

This is the green port trap. Businesses on ISP ATAs are on a VOIP service they don't control, can't customise, and in many cases can't easily leave (number porting from ISP ATA services can be difficult or impossible for some configurations). The service is cheap or included in the broadband bundle, but it delivers none of the call flow features (ring groups, IVR, call recording, voicemail-to-email, multiple DIDs) that a business actually needs.

Situation 2: A Standalone ATA You Actually Control

A standalone ATA is a separate device -- not built into the modem -- that you purchase and configure yourself (or have your VOIP provider configure). It connects to your router via Ethernet and has one or more phone ports (FXS ports) for analogue handsets. You enter the SIP credentials from your chosen VOIP provider into the ATA's web interface, and the analogue phone connected to it registers with your VOIP service.

This is the correct use of an ATA for business. You choose the VOIP provider. You own the ATA device. The phone number is registered to you. You can use the same VOIP provider's features (ring groups, IVR, call recording) just as if you were using a native SIP desk phone -- with one caveat: the analogue phone itself can't display caller ID lines, can't use BLF (busy lamp field) buttons, and can't do attended call transfers. It's a phone call in, phone call out device.

Popular ATA Models Available in Australia

The ATA market in Australia is dominated by Grandstream and Cisco (legacy Linksys/SPA range). Grandstream units are the most widely deployed for SMB use due to their low cost, broad VOIP provider compatibility, and solid reliability. Cisco SPA units appear frequently in legacy enterprise deployments.

Grandstream HT801 -- Single-Port Entry Level

The Grandstream HT801 is the most basic ATA in the Grandstream HT range. It provides a single FXS port for one analogue phone. Configuration is through a simple web interface accessible from the same network. It supports G.711 (ulaw/alaw), G.729, G.722, and G.726 codecs, making it compatible with all major Australian VOIP providers. The HT801 is the right choice if you have one analogue phone or fax machine to connect and nothing more. At around $50 AUD, it is the lowest barrier to testing an analogue phone on a VOIP service.

Grandstream HT802 -- The Most Common Business ATA in AU

The HT802 is the most widely deployed standalone ATA in Australian SMB settings. Two FXS ports allow two analogue devices -- most commonly a desk phone on port 1 and a fax machine on port 2. Each port registers independently with the VOIP provider and can have its own SIP extension or DID. The HT802 supports the same codec range as the HT801 plus T.38 fax passthrough, which is important for fax-over-IP reliability. At around $70 AUD, the HT802 represents excellent value for businesses migrating two existing analogue lines to VOIP.

Grandstream HT814 -- Multi-Port for Larger Analogue Deployments

The HT814 provides four FXS ports, making it appropriate for small offices with multiple analogue phones across different desks. It supports G.711, G.722, G.729, G.726, and T.38 fax, and adds support for provisioning via DHCP option 66 or a provisioning server -- useful if a VOIP provider manages the configuration centrally. At around $150 AUD, the HT814 is cost-effective for connecting 3-4 analogue phones to a VOIP service during a migration period. Note: for long-term use with 4 phone users, investing in 4 dedicated SIP desk phones ($80-130 each) often makes more sense than one HT814, as SIP phones deliver better call quality and features per seat.

Cisco SPA112 -- Enterprise-Grade 2-Port ATA

The Cisco SPA112 (a legacy product from the Linksys SPA series, rebranded under Cisco after the 2013 acquisition) is a 2-port ATA with a strong enterprise pedigree. It supports a broader codec set than most Grandstream HT units and has robust QoS marking for DSCP, which is relevant in networks with managed QoS policies. The SPA112 is often found in legacy deployments migrated from Cisco Call Manager environments. At around $80-100 AUD (mostly through secondary market and specialist distributors rather than mainstream retail), it is a solid choice but harder to source new than Grandstream units.

How to Set Up a Standalone ATA for VOIP

Setting up a Grandstream HT802 (or similar ATA) with a VOIP provider follows the same basic steps regardless of provider:

1. Connect the ATA to your router: Ethernet cable from the ATA's LAN port to any available port on your router or network switch. The ATA will obtain an IP address via DHCP.
2. Connect the analogue phone to the ATA: Use a standard RJ11 phone cable from the phone to the ATA's Phone 1 port (or Phone 2 for the second device on an HT802).
3. Find the ATA's IP address: Log into your router's DHCP client list to find the IP address assigned to the ATA, or use a network scanner.
4. Access the ATA web interface: Open a browser and go to the ATA's IP address (e.g., http://192.168.1.105). Default credentials are usually admin/admin (change these).
5. Enter SIP credentials from your VOIP provider: Navigate to the FXS Port settings. Enter the SIP Server address, SIP Username, and SIP Password provided by your VOIP provider. Also set the SIP User ID and Authenticate ID (usually the same as username).
6. Save and reboot: Apply settings and reboot the ATA. Within 30-60 seconds, the phone line should be active.
7. Test: Lift the analogue handset and listen for a dial tone. Dial a mobile number to confirm outbound calls work.

Most VOIP providers supply specific ATA configuration guides for Grandstream HT series devices -- ask for these during onboarding. If your provider doesn't offer ATA setup guidance, that's a signal to look for a more hands-on provider.

When an ATA Makes Sense for Your Business

An ATA is the right tool in three specific situations:

During VOIP Migration: Keeping Existing Phones

If your office has working analogue desk phones (Panasonic, Uniden, Telstra branded units, or older Avaya/Nortel handsets) that you want to keep using while you transition to VOIP, an ATA is the most cost-effective bridge. Rather than replacing every handset on day one, you can deploy one ATA, connect your existing phones, and start using the VOIP service immediately. Then replace handsets incrementally as they fail or as budget allows. See the landline to VOIP migration guide for the full transition workflow.

Fax Machines on VOIP

Fax machines are analogue devices and cannot connect to a VOIP service directly. An ATA with T.38 fax passthrough support (the Grandstream HT802 and HT814 both support T.38) allows a fax machine to send and receive faxes over an IP network. T.38 is the standard for fax over IP and significantly improves fax reliability compared to passing fax tones through a standard voice codec. Not all VOIP providers support T.38 -- confirm this before deploying a fax on VOIP. If fax is business-critical (legal, medical, property), verify T.38 support explicitly with your provider rather than assuming it works.

Lift Phones, Fire Panels, and Emergency Lines

Buildings with lifts, fire alarm panels, EFTPOS diallers (older models), and duress buttons often have analogue phone lines running to these devices. When transitioning a building's phone infrastructure from copper to VOIP, these legacy analogue connections need a bridge. An ATA (often the Grandstream HT814 for multi-device buildings) provides this bridge. Important caveat: lift emergency phones and fire panel diallers have specific regulatory requirements (AS 1735 for lifts, AS 1670 for fire systems). Confirm with the equipment supplier that VOIP-via-ATA meets the relevant Australian Standard before deploying. Some building certifiers and fire protection engineers require certified copper line emulation rather than standard VOIP ATAs for these applications.

When an ATA Does NOT Make Sense

ATAs are a bridge solution, not a destination. There are several scenarios where an ATA is the wrong tool:

• As the permanent main business line: Analogue phones connected via ATA cannot display caller ID from the VOIP system (usually just a number, no name from the VOIP contact directory), cannot use BLF keys to see if a colleague is on a call, cannot do attended transfers (put the caller on hold, check if the recipient is free, then transfer), and cannot use features like call park. If this is your front desk phone, invest in a proper SIP desk phone.
• When call quality is the priority: The audio path through an ATA and analogue phone involves two digital-to-analogue conversions. SIP desk phones are purely digital end-to-end. The difference is noticeable in call quality, particularly on long calls or in noisy environments. If your team spends significant time on calls, native SIP handsets deliver better audio.
• When you are scaling beyond 4 seats: Configuring and maintaining multiple ATAs across a growing team becomes operationally messy. At 5+ seats, standardising on SIP desk phones that are provisioned and managed centrally through the VOIP provider is simpler, more reliable, and easier to support.

Australian Businesses: What You Need to Know About ATAs

The NBN Silent Migration: Most AU Businesses Are on ISP ATAs Without Knowing It

When Australia transitioned from copper PSTN to NBN, the vast majority of business customers were silently migrated to ISP-delivered VOIP via the built-in ATA in their NBN modem. The phone kept working. The number stayed the same. Nothing obviously changed. But the underlying technology changed completely, and most businesses were not told they were now on a VOIP service rather than a copper line.

The consequence: millions of Australian SMBs are currently on ISP ATAs with no call flow features, no ability to add DIDs, no ring groups, no IVR, and in many cases no easy path to number portability. The PSTN copper shutdown (completed progressively as NBN rollout progressed nationally) means there is no going back to the copper option. The choice now is: stay on the ISP ATA (limited, ISP-controlled) or move to a dedicated VOIP service (full-featured, operator-controlled).

See the PSTN shutdown guide and VOIP vs traditional phone comparison for context on how Australian telephony arrived at this point.

Power Outage: ATAs Die When the Power Goes Out

Traditional copper landlines worked during power outages because the copper pair carried power from the telephone exchange. NBN connections -- and anything connected via ATA -- do not. When the power goes out, the NBN NTD (Network Termination Device) stops working, the router stops working, and the ATA stops working. Your analogue phone connected to the ATA is silent.

For businesses where telephone availability during power outages is important (medical practices, emergency response, businesses in areas with unreliable power), mitigation options include: a UPS (uninterruptible power supply) covering the NBN NTD, router, and ATA ($200-400 AUD for 30-60 minutes of backup power); and VOIP platform call diversion rules that redirect calls to a mobile number if the VOIP device fails to register. Both should be considered together. See the NBN VOIP setup guide for power resilience recommendations.

000 Emergency Calling via ATA

Calls to 000 from a VOIP line (including a phone connected via ATA to a proper VOIP service) are transmitted through the VOIP provider's infrastructure to the emergency call handler. Under ACMA requirements, Australian VOIP providers must support 000 calls. However, unlike copper landlines where your physical address is automatically transmitted to emergency services, VOIP 000 calls rely on a registered service address that you must provide to your VOIP provider. If the registered address does not match where the call is being made from (e.g., a remote worker using a softphone or a business that has moved premises without updating the service address), emergency services may dispatch to the wrong location. Always register and update your service address with your VOIP provider, and make this part of onboarding for any new phone deployment including ATA-connected phones.

Can You Port the Number from Your ISP ATA?

In most cases, yes -- but it depends on how the ISP has provisioned the service. Geographic numbers (02, 03, 07, 08) assigned to an ISP ATA service are generally portable under ACMA's local number portability rules. The receiving VOIP provider initiates the port on your behalf. The ISP (as the losing carrier) is required to release the number within the regulated timeframe (5-10 business days in most cases).

However, some ISPs have provisioned numbers in a way that makes porting difficult or creates delays: numbers allocated from a block owned by the ISP rather than a carrier-grade block, numbers tied to a bundled service agreement with exit fees, or ISP customer service teams that are unhelpful with port-out requests. In rare cases, an ISP ATA number may not be portable at all -- this is more common with very old VoiceSaver or similar legacy bundled services.

Before committing to a new VOIP provider, ask them to initiate a porting feasibility check on your existing number. Reputable providers will do this before you sign -- not after. See the guide on hidden VOIP costs for porting gotchas to watch for.

NBN Call Quality: ATA vs Native SIP Phone

NBN connection quality varies significantly by technology type and distance from the node. On a solid FTTP or HFC connection with a quality router and QoS configured, VOIP call quality through an ATA is generally excellent -- G.711 codec, full 64kbps audio, comparable to a copper landline. On an FTTN connection with a long copper run (high attenuation), or on a congested HFC network during peak hours, call quality can degrade: jitter, packet loss, and dropped calls become more common.

ATAs do not have internal jitter buffers as sophisticated as native SIP phones and high-end VOIP desk phones. A Yealink T54W, for example, has an adaptive jitter buffer that compensates for network variability. An analogue handset connected to a Grandstream HT802 relies on the ATA's simpler jitter buffer. The practical difference is small on a well-provisioned network and more noticeable on marginal connections.

If your NBN connection is FTTN or has known quality issues, investing in native SIP handsets rather than ATAs will deliver noticeably better call quality. Check the VOIP call quality guide for a full breakdown of NBN-specific quality factors.

What Most Businesses Get Wrong About ATAs

Mistake 1: Buying a New SIP Phone and Trying to Plug It Into the Green Port

This is the most common and most frustrating mistake. A business owner reads about VOIP, decides to upgrade from the basic analogue handset on the NBN modem, and buys a Yealink T31P or Grandstream GXP1625. They plug it into the green phone port on the NBN modem. Nothing happens. The phone won't register. The business owner calls the ISP, who says "we don't support third-party devices". The phone gets returned.

What went wrong: the green port on the NBN modem is not a SIP-accessible port. It is an analogue FXS port connected to the ISP's own ATA. A SIP phone cannot register with the ISP's VOIP credentials because those credentials are locked to the modem and not exposed. The SIP phone needs its own VOIP service -- a separate account with a VOIP provider (not the ISP) -- and connects to your network via Ethernet, not via the green port. The green port is for analogue phones only.

Mistake 2: Using an ATA as a Long-Term Solution for the Main Business Line

An ATA is an excellent migration tool and a practical solution for fax and legacy devices. It is not an ideal long-term solution for the primary business phone line. Analogue phones connected via ATA cannot support the features that make a VOIP system worth having: attended transfers, BLF buttons showing who is on a call, call park, multiple line appearances. For the front desk or the primary inbound line, a proper SIP desk phone delivers a fundamentally better experience for both staff and callers. The best SIP desk phones guide covers entry-level options starting at $80 AUD that outperform any ATA-connected analogue handset in business use.

Mistake 3: Not Checking Whether the ISP ATA Number Can Be Ported Before Signing With a New Provider

Businesses that want to move from their ISP ATA to a proper VOIP service sometimes sign a contract with a new VOIP provider, only to discover during the porting process that their number has a complication -- tied to a bundled plan with exit fees, or provisioned in a way that makes the port take 3-4 weeks instead of 5 business days. By this point, they are committed to the new provider and dealing with the porting problem from a position of less leverage. The fix: ask the new provider to run a porting feasibility check before you sign. Take 10 minutes to confirm the port is straightforward. If it is not, understand exactly what the delay or cost will be before committing.

Your Next Steps: Choosing the Right Path

Use this checklist to determine the right approach for your business:

• Check what's plugged into your NBN modem right now: If your phones connect to the green port on the modem, you are on an ISP ATA. Make a note of which ISP and what number.
• Decide whether to keep analogue phones or replace them: If existing phones are less than 3-4 years old and in good condition, an ATA bridge during migration is reasonable. If they are older than 5 years or basic unbranded handsets, budget for SIP replacements.
• Check porting feasibility for your number: Contact a VOIP provider and ask them to run a porting check before you commit.
• Choose a VOIP provider that supports ATA configuration: Not all do. Confirm the provider supplies SIP credentials compatible with Grandstream HT series ATAs.
• For fax machines: Confirm your chosen provider supports T.38 before deploying fax on VOIP.
• Plan for power resilience: If phone availability during outages matters, budget for a UPS covering the NBN NTD, router, and ATA.
• Verify 000 address registration: Once live on the new VOIP service, confirm your service address is correctly registered.

Not sure which setup is right for your team? Use the phone system sizing wizard to get a personalised recommendation, or get a free recommendation from the Need to Know Comms team.

What is an ATA adapter and why would I need one for my business?

An ATA (Analog Telephone Adapter) converts the audio signal from a traditional analogue phone into digital SIP data that can travel over an internet connection. You would need one if you want to connect existing analogue handsets (Panasonic, Uniden, or other corded desk phones) to a modern VOIP service without replacing the physical phones. ATAs are also used to connect fax machines, lift phones, and fire panel diallers to IP networks.

Can I use a Yealink or Grandstream SIP phone with the green port on my NBN modem?

No. The green phone port on an NBN modem is an analogue FXS port connected to an ISP-controlled VOIP service. SIP desk phones cannot register with the ISP's VOIP credentials because those credentials are locked inside the modem and not accessible to third-party devices. A SIP phone like the Yealink T31P needs its own VOIP service account (from a provider like Maxotel or similar) and connects to your network via Ethernet -- not via the green port. The green port is for analogue phones only.

What is the difference between an ISP-supplied ATA and a standalone ATA?

An ISP-supplied ATA is built into your NBN modem and configured by your ISP with their own VOIP credentials. You don't control the service, the credentials, or the call features. A standalone ATA is a separate device you purchase (like the Grandstream HT802) and configure with SIP credentials from a VOIP provider of your choosing. With a standalone ATA, you own the service, choose the provider, and can access full VOIP features (IVR, ring groups, call recording). The standalone ATA is the tool that lets you escape the ISP ATA lock-in.

Which ATA adapter should I buy for my small business in Australia?

The Grandstream HT802 (~$70 AUD) is the most practical choice for most Australian SMBs. It provides two ports (one phone, one fax), supports all major codecs including T.38 for fax passthrough, is widely compatible with Australian VOIP providers, and has a straightforward web configuration interface. If you only need one port, the HT801 (~$50) is sufficient. If you need four ports, the HT814 (~$150) covers a small office with multiple analogue lines. Grandstream HT units are available from Scorptec, PLE Computers, and various AU business IT distributors.

Does an ATA work for fax machines in Australia?

Yes, with caveats. ATAs with T.38 fax passthrough support (including the Grandstream HT802 and HT814) allow fax machines to send and receive over VOIP. T.38 is the recommended standard for fax over IP -- it encapsulates fax signals more reliably than passing raw fax tones through a voice codec. Before deploying fax on VOIP via ATA, confirm your VOIP provider supports T.38. If your fax use is business-critical (legal or medical practice), verify with the provider that T.38 is supported end-to-end and test a few fax transmissions before relying on it operationally.

Can I port my phone number from an ISP ATA to a new VOIP service?

In most cases, yes. Geographic numbers (02, 03, 07, 08) assigned to ISP ATA services are generally portable under ACMA's local number portability rules. The new VOIP provider initiates the port on your behalf and the ISP must release the number within the regulated timeframe (typically 5-10 business days). However, some ISPs have provisioned numbers in ways that complicate porting -- tied to bundled contracts, allocated from non-standard blocks, or with slow port-out processes. Always ask your new provider to run a porting feasibility check before signing a contract, not after.

Does an ATA-connected phone work during a power outage?

No. Unlike traditional copper landlines (which carried power from the telephone exchange), ATA-connected phones stop working when the power goes out. The ATA needs power to operate, and the NBN NTD and router also need power for the network connection to be available. For businesses where phone availability during outages is important, mitigation options include a UPS (uninterruptible power supply) for the NBN NTD, router, and ATA -- providing 30-60 minutes of backup power -- and call diversion rules in your VOIP platform that redirect calls to a mobile number if the VOIP devices fail to register.