Ask most businesses how many lines they need and the answer is the number of people who have a phone. That is almost always wrong, usually by a factor of three or four, and it is wrong in the expensive direction.
A channel is a concurrent call
One channel carries one call at a time, in either direction. Thirty staff do not need thirty channels, because they are never all on the phone at the same moment. What you are buying is the height of your busiest minute, not the size of your team.
Start from the data you already have
Your current provider's CDR, or the reports in your PBX, will tell you exactly what you need. Take a normal month and find the highest number of calls that overlapped at any single moment. That is your real peak.
If you cannot get concurrency directly, this gets you close: take the busiest hour, multiply the number of calls by the average call length in minutes, and divide by 60. Twelve hundred minutes of talk in an hour is twenty concurrent calls.
Use the busiest hour, not the average dayAverages hide the only number that matters. A support desk can sit at four concurrent calls all afternoon and hit eighteen for twenty minutes after a service alert — and it is those twenty minutes that decide whether callers get through.
Rules of thumb, when you have no data at all
For a new site with nothing to measure, these get you into the right range:
- General office: roughly one channel per four to five staff
- Sales floor or support desk: one per one and a half to two agents
- Outbound dialling: at least one per agent, and more if the dialler is pacing ahead of them
- Warehouse, workshop, site office: one per eight to ten staff
Then add headroom. Twenty per cent above your measured peak absorbs a busy week without you thinking about it.
What happens when you run out
Nothing dramatic, which is the problem. The call simply does not connect — inbound callers get busy tone or fall to voicemail, outbound users get an error — and nobody reports it because from the outside it looks like a blip.
Watch your rejected-call counter. A trunk that is genuinely too small shows up there long before anyone complains.
Bandwidth per channel
Voice is small. On G.711, the codec you want on a UK trunk, budget about 100 kbps per concurrent call once headers are counted — so twenty channels is around 2 Mbps in each direction.
That is nothing on a modern connection. What matters far more is that the voice traffic is prioritised so it is not fighting a backup upload for the same pipe. See why VoIP calls sound bad for what happens when it is not.
Buy for the peak you have, not the one you fear
Over-ordering is the common mistake because it feels safe, and on a fixed monthly trunk it is money that never comes back.
On a Pay As You Go arrangement the question mostly goes away: you are billed for the calls you make, so the channel count is a ceiling rather than a bill. Set it above your peak with room to spare, and revisit it when your call volume genuinely changes.
We size trunks from real call data, not headcount, and adjust as you grow.
See our voice routes
