How Many SIP Trunks Does My Business Need?
The number of SIP trunk channels you need equals your realistic peak number of simultaneous calls, not your total staff count or your total calls per day. Each channel carries exactly one call at a time, so if your business has, at most, 8 people on the phone at the same moment, 8 channels covers you — regardless of whether you employ 15 people or 150.
If you haven't come across SIP trunks before, our guide to what a SIP trunk is covers the basics first.
What Actually Determines the Number of Channels You Need?
- Peak concurrent calls — the highest number of calls happening at exactly the same time, not spread across the day
- Inbound and outbound together — both directions use the same pool of channels, so a busy period of outbound calling counts just as much as inbound
- Call queues and hold time — calls waiting in a queue still occupy a channel, so a business with a busy reception queue needs more headroom than one with instant pickup
- Any overflow or call recording features — some configurations use an extra channel briefly during transfers or recorded greetings, worth checking with your provider
A Simple Way to Estimate Your Peak Concurrent Calls
Rather than trying to calculate this from first principles, the most reliable method is to look at your actual, or realistically expected, calling pattern:
- Identify your busiest hour of the week for phone activity.
- Estimate how many calls are genuinely happening at the same moment during that hour, not how many happen across the whole hour.
- Add a reasonable buffer — commonly 20–30% — to cover unexpected peaks without every busy period causing a caller to get an engaged tone.
Businesses with an existing phone system can often pull this directly from call reporting, rather than estimating. If you don't have this data yet, a cautious estimate based on staff numbers and role is a reasonable starting point.
Worked Examples by Business Size
| Business type | Typical peak concurrent calls | Suggested channels |
|---|---|---|
| Small office, low call volume (5–10 staff) | 2–3 | 3–4 |
| Growing business with active sales team (10–25 staff) | 5–8 | 6–10 |
| Business with a staffed reception desk (25–50 staff) | 8–15 | 10–18 |
| Contact centre or high call volume operation | 20+ | Sized individually, often 25+ |
These are starting points, not fixed rules — the right number always comes back to your actual or realistically expected concurrent call pattern, not headcount alone.
What About Growth and Seasonal Peaks?
Most SIP trunk providers allow channels to be added or removed with relatively short notice, which is one of the genuine advantages over old-style physical lines that took longer to provision. It's worth building a small amount of headroom into your initial sizing rather than cutting it exactly to today's peak, particularly if you're expecting growth or know your business has predictable seasonal spikes — retail around Christmas, or education around enrolment periods, for example.
Common Sizing Mistakes
Sizing around total staff rather than concurrent calls The most frequent mistake. A 40-person office rarely has 40 simultaneous calls, so paying for that many channels wastes money on capacity that's never used.
Ignoring call queue behaviour A queue holding several callers at once uses a channel per caller, even while they're waiting rather than actively speaking — this is easy to underestimate for busy reception lines.
Not accounting for outbound alongside inbound Channels are shared across both directions, so a business with heavy outbound sales calling needs to factor that in alongside inbound customer calls, not size for one and ignore the other.
Cutting capacity too close to current peak usage Leaves no room for growth or an unusually busy day, both of which are common enough that a small buffer is almost always worth the modest extra cost.
Frequently Asked Questions
What happens if all my SIP trunk channels are in use and another call comes in? The caller typically hears an engaged tone or is routed to voicemail, depending on your configuration — which is exactly why sizing with a reasonable buffer matters.
Can I add more channels later if my business grows? Yes, this is usually straightforward with most SIP trunk providers, often without the lead time physical lines used to require.
Does call recording use up an extra channel? Not typically for the recording itself, but it's worth confirming with your specific provider, since some configurations handle this differently.
Is it cheaper to oversize slightly or risk running short? A modest buffer is usually the better trade-off — the cost of a few spare channels is generally small compared to the cost of missed calls during your busiest periods.
Getting channel sizing right avoids paying for capacity you don't need, or running short when it matters most. Request a quote for help sizing SIP trunks properly for your business, or read our guide to what a SIP trunk is if you're still weighing up whether you need one.
