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. Many Australian SIP trunk providers charge per channel or bundle a fixed number of concurrent calls into a monthly plan; some usage-based services have no fixed channel limit. Where you pay per channel, 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. Channel changes may be available remotely or through a self-service portal, but availability and activation time vary by provider and plan. 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.

A Practical Rule of Thumb for Sizing SIP Trunk Channels

A standard way to size concurrent call capacity is the Erlang B traffic model, which estimates trunk requirements from offered busy-hour traffic and a chosen blocking probability, subject to the model's traffic assumptions. For most small and medium Australian businesses, running the full Erlang B calculation is overkill. The simplified rule of thumb below gets you a reasonable working number without it, but treat it as a starting estimate to sanity-check with your provider, not a precise formula:

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. A queued external caller continues to occupy one SIP trunk channel. Transferring that caller between internal extensions does not add another trunk channel, but forwarding or transferring the call to an external number generally uses a second channel. Allow separately for queued callers and any external forwarding or transfer call legs, then validate the resulting capacity against measured busy-hour traffic and an acceptable blocking probability.
3. Add a safety buffer for spike days. An extra channel or two above the buffered number is a reasonable margin for predictable high-volume periods like Mondays after long weekends or end-of-month spikes. If your call volume is large or growing quickly, ask your provider to run a proper Erlang B calculation instead of relying on this rule of thumb.

Worked Examples for Australian Businesses

Example 1: 5-person medical reception desk. At peak, 3 staff are on calls simultaneously. Allow 1 illustrative extra channel for an additional queued caller or an external transfer; internal hold or an internal transfer does not itself add another SIP trunk 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; Two channels can support one active one-channel external call and one additional one-channel inbound call; any further headroom should reflect actual traffic and routing requirements.

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. Transferring an inbound call to another internal extension continues to use one SIP trunk channel. Transferring or forwarding it to an external number generally uses a second channel. 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. Per-channel charges and inclusions vary materially by provider; some services instead use bundled or usage-based pricing. 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. Some providers bundle a fixed number of concurrent calls; check the current plan price, call inclusions and channel limits.
• 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 can vary by provider and location, so businesses should assess service suitability at their specific sites.

Can You Add Channels Mid-Contract?

Many providers can change SIP trunk capacity remotely, but activation time, self-service availability, minimum terms and any required internet-capacity changes vary by provider. Where the existing PBX, provider service and internet connection already support the added capacity, the change may be remote; hardware or site work can still be necessary in some deployments. 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

For 3CX, compare SIP trunk capacity with the 3CX simultaneous-call licence limit. Standard FreePBX does not use the same platform-wide simultaneous-call licensing model, although trunk, configuration and system-capacity limits still apply. Buying more SIP trunk channels than your 3CX licence permits for simultaneous calls means the PBX caps concurrent calls at the licensed limit, 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 calls do not consume SIP trunk channels, but 3CX counts internal calls toward its simultaneous-call licence, so the PBX licence may need additional capacity beyond the external trunk-channel count.

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:

• Call logging and reporting may be available through your PBX or provider; confirm which call-attempt and rejection data your service exposes. Review monthly call attempt data for any instances where calls were rejected due to capacity limits (Where the relevant provider or PBX log captures capacity rejections, they may appear as failed attempts with provider- or platform-specific SIP responses or reason labels.).
• Use capacity-rejected calls as a prompt to review busy-hour traffic, your acceptable blocking probability and the business impact, then adjust the channel count accordingly.
• 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 extra capacity depends on the provider's current channel, bundle or usage-based pricing. 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.

Estimating channel requirements "just in case" can lead to over-provisioning and unnecessary capacity. An unused channel buffer creates an ongoing cost where the provider charges for fixed capacity; calculate that cost using the provider's actual pricing model. The appropriate buffer depends on offered busy-hour traffic, the number of available channels and the acceptable probability of blocking. Monitor concurrent and capacity-rejected call data across representative busy periods, then size capacity using observed offered traffic and an acceptable blocking probability. Adjust upward as your business grows -- Channel-change and downgrade terms vary: some Australian providers permit month-to-month changes, while others commit selected channels for a minimum contract term.

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 capacity-rejection statistics are unavailable, ordinary answered-call reports alone may not reliably reveal callers rejected upstream; ask the provider what failed-attempt data is available.

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, Some Australian providers will review your requirements and recommend a starting channel count based on your traffic estimates. 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 trunk pricing varies: some plans charge per channel, while others use bundles or usage-based pricing without a fixed channel fee. One simultaneous call uses one channel. Advertised Australian SIP trunk prices vary substantially by provider and inclusions: PAYG channels can cost well under $10 per month, while plans with included calls can exceed $25 per channel. The monthly cost of supporting five concurrent calls varies substantially with the provider, call inclusions, numbers and whether pricing is channel-based, bundled or usage-based; PBX costs may be separate. For context, a hosted cloud phone system (where the provider bundles PBX and SIP trunks) runs $20-35 per user per month inclusive, for a standard plan with Australian calls included, as at August 2026. Whether hosted cloud is cheaper than separate SIP trunks plus a self-hosted PBX depends on user count, concurrent calls, call usage, hosting, support and administration costs.

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.

What is the minimum number of SIP trunk channels for a small business?

If two one-channel external calls may overlap, 2 channels allow the second call to proceed while the first remains active. For a sole trader who takes calls one at a time and does not worry about missing the occasional second call, 1 channel is sufficient. Choose the starting channel count from expected peak concurrency, routing requirements and acceptable blocking risk, then validate it against actual usage.

Does a SIP trunk channel count affect call quality?

No. Provisioned channel count does not determine per-call bandwidth. A G.711 call commonly requires roughly 80-100 kbps in each direction including overhead, while compressed codecs can require substantially less; total bandwidth rises with the number of active calls. Merely increasing the provisioned channel limit does not by itself change per-call quality, but using more simultaneous channels raises aggregate bandwidth demand and can degrade calls if network capacity or QoS is inadequate. Call quality also depends on codec, bandwidth, congestion, latency, jitter and packet loss.

Do internal calls between extensions use SIP trunk channels?

No. Calls between extensions within your PBX are internal and do not traverse the SIP trunk. Only calls that go out to the public telephone network (to external mobiles, landlines, or 13/1300/1800 numbers) use SIP trunk channels. Internal extension-to-extension calls do not consume SIP trunk capacity, but they remain subject to PBX licensing and system-resource limits; 3CX counts internal calls toward its simultaneous-call licence.

What happens when all SIP trunk channels are busy?

When all permitted channels are occupied, another call may be rejected or produce a busy result. The exact SIP response depends on whether the carrier or PBX enforces the limit; it is not always 486 Busy Here. Some providers allow you to configure overflow behaviour: sending excess calls to a different number (a mobile, for example) or to voicemail. Outbound calls also fail when all channels are occupied. If this happens regularly during business hours, it is a signal that you need to increase your channel count.

How is SIP trunk pricing different from traditional phone line pricing?

Traditional ISDN lines had a fixed cost per physical circuit, required technician installation, and had long lead times for additions. SIP trunk pricing and provisioning vary by provider: capacity may be channel-based, bundled or usage-based, and scaling may be remote where the existing service, PBX, hardware and internet connection support it.

Can I have SIP trunk channels from multiple providers?

Yes, and some businesses do this for redundancy: a primary trunk for normal operations and a secondary trunk from a different provider as failover. A second provider can supply outbound failover through PBX backup routes. Maintaining inbound call continuity also requires the primary numbers to have provider-level failover, diversion or another arrangement that can deliver calls when the primary carrier is unavailable.