How Many SIP Trunks Do I Need? A Sizing Guide for Australian Businesses (2026)

The number of SIP trunks you need equals your peak simultaneous calls, not your staff count. Most small businesses need 2-4 trunks. This guide explains exactly how to calculate the right number for your business, walks through a worked example for an 8-person office, and tells you when SIP trunk sizing actually matters versus when it is irrelevant to your setup. It is written for business owners who have heard the term "SIP trunk" but are not sure how many they need or why the answer is not simply "one per person". By the end, you will be able to size your SIP trunks correctly, avoid overpaying, and avoid the embarrassing situation of callers getting a busy signal during your busiest hours.

What a SIP Trunk Actually Is (One Call Path, Not One Phone)

A SIP trunk is a single simultaneous call path between your phone system and the outside world. Think of it as a lane on a highway. Each lane carries one car at a time. If you have 3 lanes (3 SIP trunks), 3 calls can happen at the same time. A fourth caller trying to connect has to wait or gets a busy signal because all lanes are full.This is different from how most people think about phone lines. In the old landline world, each physical line was a connection. You paid for lines, not calls. SIP trunks work the same way conceptually - each trunk is one connection that can carry one active call at any moment - but they run over your internet connection rather than a copper wire.The key word is simultaneous. If your 8-person team makes 40 calls in a day but only ever has 3 calls happening at the same time, you need 3 trunks. The total call volume does not matter. The total staff count does not matter. Only the peak concurrent calls matter. This is the single most important thing to understand about SIP trunk sizing, and it is exactly where most businesses get it wrong. For more background on how SIP trunking works, see the guide on how SIP trunking works.

When SIP Trunk Sizing Matters (and When It Doesn't)

Before sizing trunks, you need to know whether trunk count is even relevant to your setup. The answer depends on what type of phone system you have.SIP trunk sizing matters most if you have an on-premise or self-hosted PBX. This includes systems like 3CX, FreePBX, Asterisk, or any hardware PBX (Yealink, Grandstream, Fanvil PBX appliances). With these systems, you manage the PBX yourself and connect it to a SIP trunk provider. You choose how many trunks to purchase, and that number becomes the hard limit on simultaneous calls. Get it wrong and callers get busy signals. Overprovision and you are paying for capacity you never use.SIP trunk sizing is largely irrelevant if you use a hosted VOIP system. Hosted VOIP providers (also called cloud phone systems or hosted PBX providers) bundle call capacity into their per-seat pricing. When you pay for 10 seats, the provider handles the SIP trunk infrastructure on their end. You do not manage trunk counts at all. If you are evaluating this type of system, the best VOIP phone system guide for small business is a better starting point.If you are not sure which category your business falls into, the fastest way to find out is to ask your current or prospective provider: "Do I manage SIP trunk counts, or is that included in my seat pricing?" If the answer is that you manage trunks, this guide applies to you.

The Sizing Formula: Peak Concurrent Calls Plus a Buffer

The telecommunications industry uses a formal method called the Erlang B formula to calculate trunk requirements. It accounts for call arrival rates, average call duration, and acceptable blocking probability (the percentage of calls that get a busy signal). For large call centres handling thousands of calls per day, this level of precision matters.For most small businesses, Erlang B is overkill. The simpler approach gives you a result that is accurate enough and takes about two minutes to calculate.

Worked Example: 8-Person Office

Consider a typical 8-person professional services business. Receptionist, two salespeople, three service staff, an office manager, and a director. Here is what their call activity actually looks like during the busiest period (Tuesday morning, 10am-11am):

• Receptionist handling one inbound call and transferring it: 1 active call
• Sales rep 1 on an outbound prospect call: 1 active call
• Sales rep 2 on hold waiting for a customer callback: 0 active calls (hold does not use a trunk in most PBX configurations)
• Service staff: typically not on the phone at the same time as sales during peak hours
• Director takes a call from a supplier: 1 active call

At peak, this office has 3 simultaneous calls. That is the number to work from. Apply the 20% buffer: 3 x 1.2 = 3.6, rounded up to 4 trunks.Most people would assume an 8-person office needs 8 trunks. The actual answer is 3-4. That is the difference between an $80/month trunk bill and a $240/month trunk bill. Over a year, that is $1,920 in unnecessary spending - for a business that never actually had more than 3 calls going at once.To run this analysis for your own business, note down the call activity at your busiest time for a few days. Most modern PBX systems show call reports or call logs that make this easy. If you do not have logs, ask your team to estimate it. Even a rough estimate based on "how many people are ever on the phone at once during morning peak" gives you a more accurate number than headcount.

The Staff-Count Trap: Why One Trunk Per Person Is Wrong

The most common mistake in SIP trunk sizing is equating trunks to staff. It is understandable: one person, one phone, one line - it sounds logical. But it does not reflect how calls actually work in a business.Most office workers are not on the phone simultaneously. In a typical small business, at any given moment only 20-40% of staff are on active calls. This is called the concurrency ratio. For an office of 10 staff, simultaneous call activity at peak is typically 2-4 calls, not 10.There are also factors that reduce the required trunk count further. Internal calls between extensions in the same PBX do not use SIP trunks at all - they stay internal to the system. A staff member on hold waiting for a transfer occupies a PBX extension but often does not hold a trunk (depending on PBX configuration). Voicemail calls from external callers do use a trunk but are typically short.The staff-count error can go in both directions. Some businesses massively overprovision (10 staff, 10 trunks, average 2 concurrent calls - paying for 8 trunks they never use). Others underprovision after trying to save money (5 staff, 2 trunks, bursting to 4 simultaneous calls during a product launch - callers getting busy signals at the worst possible time).

SIP Trunk Sizing Table by Office Size

The table below provides typical trunk recommendations by office size for businesses with standard call patterns (professional services, retail, trades, general admin). Businesses with unusually high inbound call volume - like customer support centres, booking lines, or high-volume inbound sales - should measure peak concurrent calls directly rather than using this table. These are starting-point estimates for businesses without call log data. If your PBX has a call history report, use it to find your actual peak concurrent calls for the last 30 days. That number is more reliable than any table.

What Happens When You Run Out of Trunks

When all SIP trunks are occupied, the next inbound call gets a busy signal. The caller hears a tone indicating the number is busy and typically hangs up. There is no voicemail fallback, no queue, no redirect - just a flat busy signal. In 2026, a busy signal is an unusual and jarring experience. Most callers assume something is wrong with the business and do not call back.For outbound calls, staff attempting to dial out while trunks are full get an error message from their PBX (often displayed as "no free lines" or similar). The call does not connect. The staff member has to wait and try again.The business cost of trunk exhaustion is straightforward: lost inbound calls equal lost leads. For a business that relies on phone enquiries, even a few hours of trunk saturation during a busy campaign, a product launch, or a media mention can mean dozens of callers who never get through. Most will not try again. They will call a competitor.Some PBX configurations allow overflow handling - redirecting calls to a mobile or to voicemail when all trunks are busy. This requires an additional trunk or channel to be available for the overflow destination, and it must be configured in advance. It does not work automatically when you run out of capacity.

Australian Considerations: NBN Bandwidth and Trunk Capacity

Each SIP trunk carrying a standard-quality call uses approximately 85-100 kbps of upload bandwidth. This is a hard constraint on your NBN connection, separate from the provider's trunk count you purchase.The upload speed is the limiting factor - not download. NBN plans in Australia are typically asymmetric: fast download, slower upload. Standard NBN 25 has only 5 Mbps upload. NBN 50 has 17-20 Mbps upload. NBN 100 typically provides 17-20 Mbps upload unless you are on NBN 100/20 (20 Mbps upload) or the newer 100/40 plan. For most small businesses, NBN upload speed is not the real constraint - you can run 10 SIP trunks on an NBN 50 connection without coming close to your upload ceiling. The real NBN risk factors for SIP quality are jitter (variation in packet timing) and packet loss, which are affected by how your ISP's network performs, not just your plan speed. A business on NBN 50 with a poorly managed ISP will have worse call quality than a business on NBN 25 with a quality VOIP-aware provider.If you are on NBN FTTC (Fibre to the Curb), FTTN (Fibre to the Node), or Fixed Wireless, upload speeds can vary significantly from the plan's headline figure depending on your distance from the node and local network congestion. Run a speed test with a tool that also measures jitter and packet loss (not just upload speed) before committing to a trunk count. A result of more than 10ms jitter or more than 0.5% packet loss suggests your connection will struggle with more than 3-4 simultaneous calls, regardless of what the upload speed says.

What SIP Trunks Cost in Australia

SIP trunk pricing in Australia varies by provider and plan structure. Most Australian SIP trunk providers offer either a per-channel fee (you pay for a set number of simultaneous call paths) or a metered model (you pay per minute with a pool of channels available).Typical per-channel pricing from Australian SIP providers ranges from $10-$25 AUD per trunk per month, depending on included call minutes and whether the plan includes local, national, and mobile calls. A 4-trunk plan with unlimited local and national calls might cost $60-$90 AUD per month from a mid-tier AU provider. Business-grade plans with SLAs, dedicated support, and guaranteed upload QoS sit higher.Most providers let you add trunks at any time without contract changes, so you do not need to over-provision from day one. Start conservatively, monitor your call logs for a month, and add trunks if you see calls getting blocked. This is a better strategy than guessing high and overpaying. For a full comparison of providers, see the guide to SIP trunk providers in Australia.

Common Mistakes

Mistake 1: Sizing to headcount. Buying one trunk per employee is the most common and most expensive sizing error. Concurrent call rates in typical small businesses are 20-40% of staff count. A 10-person office with 10 trunks is almost certainly overpaying by 50-60%. Count peak concurrent calls, not bodies.Mistake 2: Sizing based on "low" and hoping. The opposite error: underprovisioning to save money, then discovering the problem when a busy signal is the first thing a new customer hears. A 3-trunk setup for a 12-person office sounds financially prudent until a campaign brings in 6 simultaneous inbound calls. Build in a 20% buffer above your measured peak.Mistake 3: Not accounting for growth. SIP trunks are easy to add, but there is usually a billing cycle lag. If you hire 3 new salespeople who each make 4 calls per hour, your peak concurrent calls can jump significantly in a single month. Review your trunk count any time you have a significant change in staff who use the phone as a primary work tool.Mistake 4: Assuming hosted VOIP needs trunk sizing. If you are on a per-seat hosted VOIP plan, trunk sizing is the provider's problem, not yours. Many business owners spend time worrying about trunk counts when they are on a system that handles this automatically. Confirm your system type before spending time on this analysis.Mistake 5: Ignoring NBN upload quality. Buying the right number of trunks but running them on a congested, high-jitter connection delivers poor call quality even when calls technically connect. An AU-specific risk with NBN FTTN and Fixed Wireless connections in particular.

Your Next Steps

Here is the practical checklist to size your SIP trunks correctly:

1. Confirm your phone system type. On-premise or self-hosted PBX (3CX, FreePBX, hardware PBX)? Then trunk sizing applies to you. Hosted VOIP on a per-seat plan? Skip the sizing exercise.
2. Find your peak concurrent calls. Pull your PBX's call history report and look for the highest simultaneous call count in the last 30 days, or observe manually during your busiest period. If you have no data, start with the sizing table above as a baseline.
3. Apply the buffer. Multiply your peak concurrent calls by 1.2 and round up. That is your trunk count.
4. Check your NBN upload speed and quality. Run a test that shows jitter and packet loss, not just upload speed. If jitter is over 10ms or packet loss is over 0.5%, address that before adding more trunks.
5. Start lean and review. Set your trunk count based on measured peak, not on a generous guess. Review your call logs after 4 weeks and adjust if needed. Most providers let you add trunks without penalty.
6. Use the calculator. Run your numbers through the phone lines calculator tool for a guided sizing check.
7. Talk to your provider. Ask your SIP provider to confirm the right number of trunks based on your usage profile. Good providers will advise you honestly rather than pushing you to overprovision.

If you are still at the stage of choosing a system, not yet sizing trunks for an existing one, see the guide to SIP trunk providers in Australia for a full comparison of Australian providers, plan structures, and pricing.

What is a SIP trunk?

A SIP trunk is a single simultaneous call path that connects your phone system (PBX) to the outside telephone network over the internet. Each trunk handles one active call at a time. If you have 4 SIP trunks, up to 4 calls can happen simultaneously. A fifth caller either gets a busy signal or waits in a queue, depending on your PBX configuration. SIP trunks replace the physical phone lines that businesses used to run from the telephone exchange.

How many SIP trunks does a 10-person office need?

Most 10-person offices need 3-5 SIP trunks. The exact number depends on how many staff are on the phone simultaneously during your busiest period, not total headcount. A typical 10-person professional services office has 2-4 simultaneous calls at peak. Add a 20% buffer and round up: a peak of 3 concurrent calls means 4 trunks. A peak of 4 means 5 trunks. Use your PBX call logs to find your actual peak, or use the starting estimate of 4 trunks and adjust after a month of monitoring.

Can I add more SIP trunks later?

Yes. Most SIP trunk providers in Australia allow you to add channels at any time, often mid-billing-cycle. There is typically no contract change required. This means you do not need to over-provision from the start. A better approach is to start with a conservatively sized number based on measured peak calls, run it for 4-6 weeks, check your call logs for any blocked calls, and add trunks if you see evidence of saturation. Starting lean and scaling up is cheaper than overpaying from day one.

Do I need SIP trunks if I have a hosted VOIP system?

No. If you are on a hosted VOIP system (also called a cloud phone system or hosted PBX), the provider manages SIP trunk capacity as part of your per-seat pricing. You pay for seats, not trunks. The provider ensures enough call capacity is available for your account. You do not need to count trunks or manage them at all. SIP trunk sizing only applies if you run your own on-premise or self-hosted PBX (such as 3CX or FreePBX) and connect it to a separate SIP trunk provider.

How much does a SIP trunk cost in Australia?

SIP trunk pricing in Australia typically ranges from $10-$25 AUD per trunk per month, depending on the provider and what call minutes are included. A 4-trunk plan with unlimited local and national calls costs approximately $60-$90 AUD per month from mid-tier Australian providers. Enterprise-grade plans with SLAs and dedicated support cost more. Most providers include some included minutes per trunk and then charge per-minute rates for excess calls. See the full breakdown in the guide to SIP trunk providers in Australia.

What is the Erlang B formula and do I need it?

Erlang B is a mathematical formula used by telecommunications engineers to calculate the number of call paths needed to handle a given volume of calls with an acceptable blocking probability (the percentage of calls that get a busy signal). It accounts for call arrival rates, average call duration, and target service levels. For large call centres or businesses handling hundreds of calls per hour, Erlang B gives precise results. For most small businesses with straightforward call patterns, the simpler approach of measuring peak concurrent calls and adding a 20% buffer is accurate enough and takes minutes rather than an engineering analysis.

Does NBN affect how many SIP trunks I can use?

Upload bandwidth is a factor, but it is rarely the limiting constraint for small businesses. Each SIP trunk uses roughly 85-100 kbps of upload bandwidth. An NBN 50 connection with 17 Mbps upload can theoretically support over 150 simultaneous SIP calls. The more relevant NBN issue for SIP trunking is call quality: high jitter (over 10ms) or packet loss (over 0.5%) will cause audio problems even on a fast connection. NBN FTTN and Fixed Wireless connections are more susceptible to these quality issues. Test jitter and packet loss, not just upload speed, before setting your trunk count.