SIP trunk channels (or ports) are the number of calls a route can carry at the same time; CPS (calls per second) is how many new calls you can start each second. A route needs enough of both: channels for your peak concurrent calls, and CPS for how fast your system dials. Calls above either limit are rejected.
Channels: concurrent calls
One channel carries one call in progress. If your peak is 200 calls at once, you need at least 200 channels, plus a margin. Channels count from the moment the call is set up, so a ringing call also takes a channel. You can estimate peak channels from minutes:
Peak channels = (minutes in the busiest hour) / 60. For example, 9,000 minutes in the busiest hour means about 150 calls in progress on average during that hour. Add 20 to 30% for peaks inside the hour.
CPS: call attempts per second
CPS limits how fast new calls arrive, whether they are answered or not. It is the limit that matters most for automated traffic. A team of 50 agents dialing by hand rarely exceeds 1 or 2 CPS. A predictive dialer calling a list can easily send 20 or more attempts per second, most of which fail or go unanswered. The CPS limit varies a lot between routes and is agreed per customer, so always ask for it in numbers before you send automated traffic.
| Traffic | Channels needed | CPS needed |
|---|---|---|
| Conversational retail, long calls | High relative to attempts | Low |
| Call center, agents dialing | About 1 to 1.5 per agent | Low to medium |
| Predictive dialer | 2 to 3 per agent | High |
| Voice OTP and short notifications | Low (calls are short) | Bursty, can be high |
See VoIP traffic types and dialer traffic routes for why carriers watch CPS closely.
What happens when you exceed the limits
When you go over channels or CPS, the seller's switch rejects the extra calls, often with a SIP 503 (Service Unavailable) or another rejection code, depending on the platform. Your switch may then fail over to another route, which raises post-dial delay, or the call fails. In your statistics this shows as lower ASR on the route that hit its limit. Ask the seller which response code they use so your LCR can react properly.
Sizing a new route
- Take your busiest hour from recent CDRs: minutes and call attempts.
- Channels: minutes divided by 60, plus 25% margin.
- CPS: attempts in the busiest minute divided by 60, plus margin. Check the busiest second too if your dialer sends bursts.
- Set your own switch to cap traffic below the agreed limit, so you control the rejection instead of the seller.
Shared and dedicated capacity
On many routes, channels and CPS come from a pool shared by all customers of the seller. At busy times you may get less than expected. Some carriers sell dedicated capacity reserved for you. See dedicated vs shared ports.
Include capacity in your request
Stating your channels and CPS up front lets the seller confirm the route can take your traffic. Read what to include in a route request, then request a voice route for VoIP termination with your numbers.
Frequently asked questions
Is a SIP channel the same as a phone line?
In practice, yes: one channel carries one call in progress, as one phone line did. The difference is that SIP channels are a limit set in software on the trunk rather than physical lines, so one internet connection can carry hundreds of channels if it has enough bandwidth, and capacity changes need no new wiring.
How much bandwidth does one SIP channel need?
About 80 to 87 kbit/s in each direction per call with G.711, and about 24 to 31 kbit/s with G.729, including packet headers. Multiply by your peak channels and add headroom. For example, 300 concurrent G.711 calls need roughly 26 Mbit/s in each direction, while the same number of G.729 calls need under 10 Mbit/s.
Does every SIP trunk have a CPS limit?
Almost always. Wholesale carriers set a CPS limit per customer to protect their switches and upstream routes, and calls above it are rejected. The figure varies widely between routes and is agreed per customer, so ask for it in numbers before sending automated or dialer traffic, and cap your own switch just below it.
What is an Erlang in call capacity planning?
An Erlang is a unit of telephone traffic: one Erlang is one channel busy for a full hour. Minutes in the busiest hour divided by 60 give the Erlangs, which equals the average number of calls in progress. Erlang B tables or calculators then add the extra channels needed to keep blocked calls below a target.
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