In short: You need one SIP channel for each external call happening at the same time at your busiest moment, plus headroom. Find your peak concurrent calls from call logs, add queues, outbound campaigns and conference calls, then check bandwidth: Cisco's figures are about 87.2 kbps per G.711 call and 31.2 kbps per G.729 call, in each direction.
What a SIP channel is
A SIP channel carries one external call at a time, inbound or outbound. Ten channels means ten people can be on outside calls at once; the eleventh caller hears engaged, or overflows to another route if you have set one up.
Channels are about concurrency, not users. An office of 40 people where staff spend little time on the phone may need far fewer channels than a 15-seat sales team that is on calls all day. Internal calls between extensions on your own PBX do not use SIP channels. See SIP trunks for the service itself.
How to size from peak concurrent calls
- Pull call logs from your PBX or current provider for a typical busy month, ideally including your seasonal peak.
- Find the busiest hour and the highest number of calls in progress at the same moment, not the total number of calls.
- Add call queues. Callers waiting in a queue for your PBX hold a channel while they wait, even before an agent answers.
- Add outbound activity: sales campaigns, callbacks or appointment reminders that run at the same time as inbound peaks.
- Add conference calls dialled out from the PBX, where each external participant uses a channel.
- Add forwarded calls. A call forwarded from the PBX to a mobile uses two channels: one in, one out.
- Add headroom for growth and unusual days, and check whether your provider offers burst capacity.
A worked example in words. Suppose call logs show that, at your busiest moment, eight people were on outside calls and four callers were waiting in a queue. That is twelve concurrent calls. If two calls were forwarded to mobiles at that moment, add two more for the outbound legs, making fourteen. Adding headroom for growth, you might size at sixteen to eighteen channels. Your own numbers will differ; the method is what matters.
For contact centres with heavy queueing, traffic engineering models such as Erlang B can help, and we can work through them with you.
Bandwidth per call by codec
Every channel in use needs bandwidth in both directions at once. Cisco publishes per-call bandwidth figures for common codecs at the default 20 ms packetisation (50 packets per second):
| Codec | Codec bit rate | Bandwidth per call including Ethernet overhead (Cisco figure) |
|---|---|---|
| G.711 | 64 kbps | 87.2 kbps |
| G.722 (64 kbps) | 64 kbps | 87.2 kbps |
| G.729 | 8 kbps | 31.2 kbps |
These figures apply to each direction, so you need the capacity on both upload and download. To estimate, multiply the per-call figure by your peak channels. Using Cisco's G.711 figure, sixteen simultaneous calls would need about 1.4 Mbps each way before any other traffic.
Overheads vary with the network type and with encryption, and adaptive codecs such as Opus (used by many apps) change their bit rate with the connection, so treat these figures as a planning baseline rather than a guarantee. Read hosted VoIP for QoS and call quality.
SIP channels vs 3CX simultaneous calls
3CX licenses its commercial editions by simultaneous calls (SC), which is a different measure from SIP channels:
| SIP channels | 3CX simultaneous calls | |
|---|---|---|
| What is counted | External calls on the trunk | Calls through the 3CX system, internal and external |
| Internal extension-to-extension calls | Not counted | Counted |
| Set by | Your SIP trunk provider | Your 3CX licence |
So a 3CX system may need a larger SC licence than the number of SIP channels on its trunk. Idle extensions do not use SC capacity, and each 3CX SC size comes with recommended and maximum user limits. Read 3CX pricing and licensing for how it works, and 3CX editions for what each includes.
Common sizing mistakes
- Sizing by headcount. Users do not equal channels; concurrency does.
- Using daily averages. A quiet afternoon hides the Monday morning peak.
- Forgetting queues and forwards. Both hold channels that are easy to miss.
- Ignoring upload. The connection, not the trunk, is often the real limit.
- No overflow plan. Decide what happens to call number seventeen: another queue, voicemail or a divert.
Send us your call logs and we will size the channels and check your connection. Contact us.
Why choose your cloud phone system with Forever Systems
We are vendor-neutral. Forever Group supplies and supports several cloud phone platforms, so we recommend the one that fits how your organisation works rather than the one we happen to sell.
- Side-by-side comparison of hosted 3CX, Microsoft Teams Phone, Forever UC, Forever Teams and Webex Calling for your users
- Contact centre and call recording options matched to your call volumes and CRM
- Number porting, handsets and set-up handled for you, including the move off analogue and ISDN lines
- Connectivity checked so call quality is right from day one
- UK support for moves, adds, changes and faults
- For BT Cloud Voice, see our BT Business Partner site cloudvoice.cloud
We work with businesses, schools, charities and public sector organisations. We do not sell consumer phone services.
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Contact usFrequently asked questions
How many SIP channels do I need per user?
There is no fixed ratio. You need one channel per simultaneous external call at your busiest time, so size from call logs rather than headcount. Teams on the phone all day need more channels per user than office staff who make occasional calls.
Does a call on hold use a SIP channel?
Yes. An external call on hold or waiting in a queue on your PBX stays connected, so it keeps using a channel until it ends.
How much bandwidth do I need for 10 SIP channels?
Using Cisco's published figure of about 87.2 kbps per G.711 call, ten simultaneous calls need roughly 872 kbps in each direction, plus headroom for other traffic. With G.729 the figure is about 312 kbps each way.
Are SIP channels the same as 3CX simultaneous calls?
No. SIP channels count external calls on your trunk. 3CX simultaneous calls count every call through the 3CX system, including internal calls between extensions.
What happens if I run out of SIP channels?
The next caller typically hears engaged, or the call follows any overflow route you have set, such as a second trunk or voicemail. Some providers offer burst capacity for short peaks.