Size SIP capacity by simultaneous calls, not by the total number of employees. Count the highest expected inbound and outbound calls during the busiest period, include queued or transferred calls where the platform uses a channel, then add sensible headroom and monitor real usage after launch.
A channel means an active call path
A SIP channel is commonly used as shorthand for one simultaneous call. The precise accounting depends on the provider and platform. 3CX describes its trunk simultaneous-call limit as the combined inbound and outbound calls using that trunk at a given time.
UKDDI plans can also define separate inbound and outbound caps. The effective capacity is always constrained by the lowest applicable limit across the provider, PBX licence, trunk configuration, SBC and network.
Start with the busiest 15 to 30 minutes
Do not divide staff by a generic ratio without checking behaviour. A ten-person office may need only two calls at once, while a four-person appointment team may regularly need four or five.
Review call records where available. Count active inbound and outbound calls during the busiest interval, then consider calls waiting in queues, transfers, conferences, recordings and external diversions.
- Peak inbound calls being answered or queued.
- Peak outbound calls made at the same time.
- Transferred calls that temporarily use more than one call leg.
- Conference calls and external participants.
- Calls forwarded back to the public network.
- Expected growth and seasonal peaks.
Practical starting examples
A sole trader receiving occasional calls may begin with one inbound call path, provided missed-call handling is acceptable. A small office with several users commonly needs at least two simultaneous calls so one conversation does not block every other caller.
A busy sales or support team should size from actual busy-hour records. If five calls are regularly active and another two callers wait, a plan capped at two inbound calls is unsuitable even if the business has only a few employees.
Examples are planning prompts, not guarantees. Measure your own call pattern and confirm how the provider and PBX count each call leg.
Add headroom without buying blindly
Allow capacity for growth, staff overlap, campaigns and unexpected incidents. Too little capacity creates busy responses or rejected calls. Excessive capacity wastes money and can increase fraud exposure if controls are weak.
A sensible process is to choose a defensible starting tier, enable monitoring and review peak utilisation after the first month and after major campaigns.
Channels also affect bandwidth and firewall design
Voice quality depends on latency, loss, jitter and the codec used, not just headline broadband speed. The firewall and RTP port range must also support the maximum simultaneous calls.
3CX notes that each call uses RTP resources and provides port-planning guidance. Use quality-of-service controls where appropriate and provide resilient connectivity for business-critical calling.
UKDDI SIP trunk concurrency tiers
Choose a Starter, Regular or Plus SIP trunk tier and add a PAYG or inclusive call package.
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