How Many SIP Trunks Does My Business Need in Australia? (2026)

A SIP trunk channel is a single simultaneous call path. When you have 4 SIP trunk channels and 5 calls arrive at the same time, the fifth caller gets a busy signal or is diverted to whatever overflow destination you have configured. phone line providers charge per channel (concurrent call capacity), per month, so buying more channels than you need is a straightforward ongoing cost leak.

The right number of SIP trunk channels for a business is not the same as the number of staff, the number of extensions, or the number of phone lines you used to have. It is the number of concurrent calls you expect during your busiest periods, with a safety buffer. This guide explains how to calculate that number for an Australian business.

What a SIP Trunk Channel Actually Is

A SIP trunk connects your PBX (phone system) to the public telephone network via the internet. Each "channel" on the trunk is one active call path. If your SIP trunk has 6 channels, 6 calls can be active simultaneously: some inbound, some outbound, some internal calls that are being transferred to external numbers.

The key distinction from traditional phone lines is that SIP channels are not physical: they are logical paths on a single internet connection. You can start with 4 channels and add more at any time, usually within minutes through your provider's portal. You are not waiting for a technician to install new copper. This flexibility is one of the main reasons businesses move to SIP from legacy ISDN or copper lines.

For a fuller explanation of how SIP trunking works and how it connects to your phone system, see our Australian phone line providers guide.

The Formula for Sizing SIP Trunk Channels

The standard formula for sizing concurrent call capacity is based on the Erlang B traffic model used by telecoms engineers. For most small and medium Australian businesses, you do not need to run the full Erlang calculation. A practical three-step approach gives an accurate enough result:

1. Count your current peak concurrent calls. If you have phone system call logs, pull the busiest 30 minutes of any weekday over the last month. Count how many calls were active at the same time at the peak moment. If you are starting fresh with no logs, estimate: how many staff handle calls simultaneously at the busiest point in your day?
2. Add a buffer for overflow and transfers. Calls being transferred between extensions or waiting in a hold queue also consume a channel. Add 20% to 30% to your peak concurrent number to account for this.
3. Add a safety buffer for spike days. Add 1 to 2 extra channels above the buffered number. This handles Mondays after long weekends, end-of-month spikes, and other predictable high-volume periods.

Worked Examples for Australian Businesses

Example 1: 5-person medical reception desk. At peak, 3 staff are on calls simultaneously. Transfers and hold add roughly 1 extra channel. Buffer for spike: 1. Total: 5 SIP trunk channels. Starting with 4 and monitoring is a reasonable approach, with plans to scale to 6 if call volumes grow.

Example 2: 15-person professional services firm. Observed peak: 6 concurrent calls during Tuesday morning billing run. Buffer (25%): 1.5, round to 2. Safety buffer: 2. Total: 10 SIP trunk channels. This may feel over-provisioned, but with most providers charging $5 to $15 per channel per month, 10 channels costs $50 to $150/month, and a busy signal to a client at a critical moment costs more than that.

Example 3: 3-person trades business. Usually only 1 or 2 calls at a time. Owner, scheduler, and one field worker. Peak concurrent: 2. Buffer: 1. Safety: 1. Total: 4 SIP trunk channels. Some providers offer a minimum of 2 channels; 4 is the sensible floor for any business that wants to handle an inbound call while someone else is already on a call.

What Drives the Channel Count Higher Than Expected

Several factors push the required channel count above what a simple staff-count estimate suggests:

• IVR and auto-attendant. A caller navigating your auto-attendant occupies a channel while the IVR plays and waits for a key press. If you have high call volumes hitting your auto-attendant simultaneously, the IVR itself consumes channels.
• Music on hold and call queues. A caller waiting in a queue with hold music playing is occupying a channel. With 10 callers queued, you need 10 channels just for the queue, plus more for the agents handling calls.
• Call transfers and internal routing. A call being transferred from reception to a team member occupies two channels briefly (one from the inbound SIP path, one for the internal transfer). High-transfer environments need more headroom.
• Outbound call campaigns. If your business makes outbound calls simultaneously with inbound handling (e.g., a sales team calling leads while support handles inbound), the outbound calls compete for the same SIP trunk channels.
• Ring groups. When a ring group is ringing multiple extensions simultaneously for an inbound call, each extension being rung occupies the same inbound SIP channel. The channel is not multiplied by the ring group size, but the call stays open until answered or timed out, keeping the channel occupied.

How Australian phone line providers Charge for Channels

Australian phone line providers typically charge for concurrent call capacity in one of three ways:

• Per-channel monthly fee: A fixed monthly cost per channel of concurrent capacity, regardless of call volume. Common pricing: $5 to $15 per channel per month. Suitable for businesses with predictable call volumes.
• Bundled channel plans: A set number of channels included in a plan, with call minutes bundled or separately charged. Common for small business plans: 4 channels included in a plan at $30 to $60/month.
• Pay-as-you-go (unlimited channels, per-minute billing): No channel cap; you pay only for the minutes of calls made and received. The channel capacity scales dynamically. Suitable for businesses with variable or unpredictable call volumes, or for protecting against busy signals during spikes.

For a comparison of these models across Australian phone line providers and their per-minute pricing, see our phone line providers guide. Call quality and channel reliability also vary by provider, particularly for businesses in regional areas where some providers have better network peering.

Can You Add Channels Mid-Contract?

With all major Australian phone line providers, yes. SIP trunk channels are provisioned through a portal and additional channels are active within minutes. There is no hardware to install and no technician visit required. This is a significant advantage over ISDN or copper lines, where adding capacity required ordering additional circuits with lead times of days to weeks.

If your business is seasonal (retail, accounting, tourism), the practical approach is to start at your off-peak channel requirement and scale up before your busy season, then scale back down afterwards. Check whether your provider charges a minimum term for additional channels or whether you can add and remove them month to month.

The one exception is porting and number allocation: if you are bringing new geographic numbers to the SIP trunk, those have a porting process with a timeline that is independent of the channel scaling.

SIP Trunk Channels and Your PBX Licence: Keeping Them Aligned

If you are running 3CX or FreePBX, the SIP trunk channel count must align with your PBX's simultaneous call (SC) licence limit. Buying 10 SIP trunk channels but running a 6-SC 3CX SMB licence means the PBX caps at 6 concurrent calls regardless of how many SIP channels are available. The excess SIP trunk capacity is wasted.

Conversely, a 12-SC PBX licence with only 6 SIP trunk channels means your PBX can theoretically handle 12 concurrent calls internally, but only 6 can involve an external party at any one time. Internal call capacity is not limited by the SIP trunk, so this configuration makes sense if your business has significant internal call traffic (large team, frequent internal transfers) alongside external call handling.

When sizing both together: start with the external call peak (simultaneous calls to or from external numbers), size your SIP trunk channels to that number plus a buffer, then ensure your PBX licence SC count matches or exceeds the SIP trunk channel count. For a full breakdown of 3CX licence sizing and costs, see our 3CX pricing guide for Australia.

Monitoring and Adjusting Over Time

SIP trunk channel requirements change as a business grows. A business that starts with 4 channels and adds staff, expands to new locations, or launches outbound call campaigns will reach the channel ceiling eventually. The practical approach is to set a monitoring trigger rather than guessing ahead of growth:

• Most phone line providers include call logging and reporting. Review monthly call attempt data for any instances where calls were rejected due to capacity limits (these appear as failed call attempts with a "busy" or "no circuit" reason code).
• If you see more than 5 to 10 capacity-rejected calls in any month, that is a signal to add 2 channels above your current count.
• For rapidly growing businesses, review the channel count quarterly and adjust proactively rather than waiting for customer-facing busy signals to trigger a review.

The cost of adding 2 extra channels proactively is typically $10 to $30 per month. The cost of a potential client receiving a busy signal at a critical moment is harder to quantify but almost certainly higher. See our guide to VOIP call quality in Australia for related performance monitoring guidance.

What Most Businesses Get Wrong

Equating SIP channels to the number of staff. A 15-person team where only 4 people handle calls does not need 15 SIP trunk channels. Buying channels based on headcount rather than observed concurrent calls leads to paying for capacity that is never used. Start with an honest assessment of your actual peak call concurrency.

Starting with too few channels to save money and not monitoring the busy signal rate. Some businesses start with 2 channels to minimise cost and discover months later that callers have been hitting busy signals at peak times. If your phone line provider offers call attempt logging or busy signal statistics, check them. If they do not, your call reports should show a drop in answered calls during your busiest periods if you are at capacity.

Forgetting to account for the SIP trunk channel capacity when sizing PBX licences. If you use 3CX, the simultaneous call (SC) count in your 3CX licence must also match or exceed your SIP trunk channel count. Buying 10 SIP trunk channels but running a 6-SC 3CX licence means the PBX caps at 6 concurrent calls, making 4 of your SIP trunk channels unusable. Align the two numbers.

Your Next Steps

To determine the right channel count for your business:

1. Pull your call logs for the last month and identify the busiest 30-minute window on any weekday. Count concurrent calls at peak.
2. If no logs are available, estimate based on how many staff handle calls simultaneously at the busiest point of your day.
3. Add 25 to 30% to the peak for queue and transfer overhead.
4. Add 1 to 2 channels safety buffer above that.
5. Check whether your PBX licence SC count matches or exceeds the channel count you land on.
6. Start with that number and review actual call concurrency after one month. Adjust up if you see any busy-signal-era call drops.

If you are unsure where to start, most Australian phone line providers will review your requirements and recommend a channel count based on your business description and estimated call volume. Use that as a starting point, then adjust based on actual usage data after the first month of operation. The flexibility to add channels without a service call is one of the clearest advantages of SIP over legacy ISDN or copper line infrastructure.

The right SIP trunk channel count depends partly on which PBX platform you are running alongside it. Our guide to the best phone system for small business Australia compares cloud phone system (where channels are managed by the provider) against self-cloud phone system with a separate SIP trunk, and covers the channel sizing implications of each approach for small Australian businesses.

What Do SIP Trunks Cost in Australia?

SIP trunks are priced per channel — one simultaneous call per channel. Dedicated SIP trunk services from AU providers typically cost $10-25 per channel per month (inc. GST). A small business using 5 channels pays $50-125 per month for the trunks alone, plus the cost of a PBX to run alongside them. For context, a hosted cloud phone system (where the provider bundles PBX and SIP trunks) runs $20-35 per user per month inclusive. For businesses with fewer than 20 users, hosted cloud is almost always cheaper than separate SIP trunks plus a self-hosted PBX.

If you need a plain-English explanation of why your business needs more than one phone line before working through the channel calculation, our guide to getting a second phone line for your business covers the basics: what a concurrent call channel is, how the engaged tone problem works in practice, and what the options are for a small team.