“How many SIP channels do I need?” is one of the first questions when a business moves to a SIP trunk. Buy too few and callers hear a busy tone at the worst moment. Buy too many and you pay every month for lines that sit idle. The good news is that this is a solved problem: with your busy hour call count, the average call length and the Erlang B formula, you can size a SIP trunk in a few minutes. This guide shows how, with worked examples and the bandwidth each channel needs.
What a SIP channel is
A SIP channel is one call at a time. A trunk with 10 channels can carry 10 simultaneous calls, inbound and outbound together. The 11th call either gets a busy signal or, if your PBX is set up for it, moves to another trunk.
Channels are not the same as phone numbers or extensions. You might have 100 extensions and 20 DID numbers on a trunk with 15 channels, because people are rarely all on the phone at once. If you are new to SIP trunks, start with SIP trunks in India and what is SIP.
The three numbers you need
- Busy hour calls: how many calls start in your busiest hour of a normal busy day, inbound and outbound together. Take it from your call reports or operator bills, not from a quiet week.
- Average call length: in minutes, including ring time on outbound calls. Many offices sit between 2 and 4 minutes; support desks often run longer.
- Grade of service: the share of calls you accept may find all channels busy in that hour. 1% (one call in a hundred) is a common target for business lines; 2% is acceptable for some internal or low-priority traffic.
Traffic in Erlangs is simply: busy hour calls × average minutes ÷ 60. One Erlang is one channel kept busy for a full hour.
Erlang B in plain words
Erlang B tells you the chance that a new call finds every channel busy, for a given traffic load and number of channels. It assumes callers who get a busy tone do not wait in a queue on the trunk, which is how trunks behave. (Call centre queues use a different formula, Erlang C, covered in Erlang C call centre staffing.)
You do not need to do the maths by hand. Any Erlang B calculator or spreadsheet gives the result. What matters is reading it correctly.
Worked example: a 60-person office
A 60-person office makes and receives about 120 calls in its busiest hour, averaging 3 minutes each.
- Traffic = 120 × 3 ÷ 60 = 6 Erlangs.
| Channels | Chance a call finds all channels busy |
|---|---|
| 10 | about 4.3% |
| 11 | about 2.3% |
| 12 | about 1.1% |
| 13 | about 0.5% |
| 14 | about 0.2% |
For a 1% target, 12 channels is just over the line, so 13 channels is the answer. Notice that the office needs about twice its average load, not the number of staff.
Worked example: a busy sales and support floor
A company with a sales team and a support desk handles 360 calls in the busy hour, again averaging 3 minutes.
- Traffic = 360 × 3 ÷ 60 = 18 Erlangs.
| Channels | Chance a call finds all channels busy |
|---|---|
| 24 | about 3.5% |
| 26 | about 1.7% |
| 27 | about 1.1% |
| 28 | about 0.7% |
| 30 | about 0.3% |
At a 1% target, this site needs 28 channels; at 2%, 26 would do. Larger groups are more efficient, so the extra channels above the average load shrink as traffic grows.
Adjust for real life
Erlang B gives a starting point. Adjust it for how your business actually behaves:
- Peaks inside the hour: if calls bunch up (a campaign email, a school result day, the first hour after a holiday), size for that peak, not the hourly average.
- Growth: add headroom for the next 12 months of hiring.
- Recorded announcements and IVR time: a caller sitting in your IVR still uses a channel.
- Call centre queues: callers waiting in a queue hold a channel while they hear music on hold. Size the trunk for agents plus the longest queue you allow.
- Branches on the same trunk: add their busy hour calls together only if their busy hours overlap.
Bandwidth per channel
Each channel needs internet bandwidth in both directions. Approximate figures per call, including packet overhead on Ethernet:
| Codec | Bandwidth per call, each direction |
|---|---|
| G.711 | about 87 kbps |
| G.722 (HD voice) | about 87 kbps |
| G.729 | about 31 kbps |
So 28 channels on G.711 need about 2.4 Mbps up and 2.4 Mbps down for voice alone, and about 0.9 Mbps each way on G.729. Leave plenty of headroom for data traffic, and see VoIP codecs explained and VoIP QoS and voice VLAN to keep voice quality steady.
Setting channel limits in Voxdial IP PBX
On Voxdial IP PBX, each trunk has a Maximum Channels setting: the most simultaneous calls the trunk may carry (blank means unlimited). Set it to what you pay for, so the PBX knows when the trunk is full instead of letting the operator reject the call.
Outbound routes list trunks in order, and the Continue if Busy option tells the PBX to try the next trunk when one is full. That lets you run a main SIP trunk plus a smaller second trunk or a PRI line as overflow. The least cost routing guide shows how to plan the order.
Signs you have too few channels
- Callers say they got a busy tone while staff were free.
- Outbound calls fail at busy times with “all circuits busy” or a fast busy tone.
- Missed and abandoned call reports spike in the same hour every day.
Check your busy hour, rerun the numbers and add channels before buying more agents. The missed call management guide explains how to separate trunk problems from staffing problems.
Not sure how many channels your business needs? Talk to BitVoice and we will size it from your own call data.
FAQs
How many SIP channels do I need for 50 users?
It depends on calls, not users. A typical 50-person office with around 100 calls of 3 minutes in the busy hour has 5 Erlangs of traffic and needs about 11 channels for a 1% chance of busy.
What is the difference between Erlang B and Erlang C?
Erlang B sizes lines where blocked calls are lost, such as SIP trunk channels. Erlang C sizes agents where callers wait in a queue, such as a call centre.
Do incoming and outgoing calls share SIP channels?
Yes, unless your operator splits them. Add inbound and outbound busy hour calls together when you size the trunk.
How much bandwidth does one SIP channel use?
About 87 kbps each way with G.711 or G.722, and about 31 kbps each way with G.729, including network overhead.
