How many simultaneous calls do I need or get on a SIP trunk?

A SIP trunk's capacity is counted in calls in progress at the same moment. Each one occupies a channel, which we define in what a SIP trunk channel is. The phone numbers you own and the people you employ don't decide the count. It depends on how many outside calls are in progress at once when the phones are busiest.

In an April 2023 post about its SIP trunks, Atlantech Online mentions a local government customer that adds capacity on its trunks during weather events such as snowstorms and tropical storms. What drives that customer's capacity is the volume of calls an event brings, so the peak is the thing to measure.

Traffic engineers measure that peak in erlangs. Over a one-hour measurement, one erlang is one hour of calls. Westbay Engineers' guide to sizing a trunk group gives the arithmetic: multiply the calls in the busiest hour by the average call length in seconds, then divide by 3,600. Its example is 350 calls averaging 180 seconds, which comes to 17.5 erlangs. Put another way, that workload is the same as 17.5 calls in progress on average across the hour.

The average is not enough capacity. Calls do not arrive evenly, so the number in progress swings above and below the average during the hour.

The Erlang B model accounts for those swings. You give it the busy-hour traffic and a blocking target, which is the share of calls you accept will find every path in use. Westbay calls 1 percent "a normal figure to use in traffic engineering" and says some applications use a looser 3 percent. Westbay's guide runs 17.5 erlangs and a 1 percent target through its Erlang B calculator and gets 27 lines. A line here means one voice path, the same thing a SIP provider calls a channel.

So the model sets 27 paths against an average of 17.5 calls. The extra paths are there for the moments when calls bunch up. The 1 percent is the model's prediction under its own assumptions. It is a planning input, and nobody has promised it to you as a service level.

Westbay attaches two cautions to the method. The first concerns where the traffic figure comes from. The model wants offered traffic, which is every call people attempted. Westbay warns that call loggers measure carried traffic, the calls that got through. Some systems also record attempts that failed, and where yours does, count them. Where it does not, a system that was already running at its ceiling shows less traffic than was offered to it, and a trunk sized from that log comes out too small.

The second caution is that random arrival is an assumption of the model. A sudden burst does not fit it. Westbay's example is the response to a radio or television advertisement, and it suggests building in extra capacity where you expect one. A storm has the same effect on a public agency's phones, which is the situation Atlantech's customer adds capacity for.

How many calls you get depends first on how your provider sells capacity.

Some providers sell it by the channel. SIP.US's pricing page lists $24.95 per channel per month on its month-to-month plan and says you need one channel for every concurrent incoming or outgoing call. That price includes calls to the lower 48 states and to Canada, excluding the Northwest Territories. Taxes, fees and phone numbers are extra. On a plan like this you get the number of calls you pay for. The 27 paths in the example would cost $673.65 a month in channel charges, and each path added as a margin costs another $24.95 a month. That makes the blocking target a spending decision as well as an engineering one.

Other providers publish no channel count. Twilio's limits page for Elastic SIP Trunking lists unlimited concurrent calls with two conditions attached. The figure depends on carrier support, and a new account without an approved Business Profile has limited concurrent calls. "Unlimited" on that page means there is no channel count to buy. The other ceilings described below still apply.

A third group quotes the trunk to fit the customer. Atlantech's business phone page describes its pricing as tailored and publishes no per-channel rate or ceiling. With a quoted trunk, ask for the simultaneous-call capacity to be stated in the quote, and give your busy-hour figure as the basis for it.

Whichever model applies, find out whether inbound and outbound calls draw on the same pool. On SIP.US's plan they do. If a plan counts the two separately, each direction has to cover its own peak.

Concurrency counts calls in progress. A second limit counts how fast new calls start. Twilio's page on calls per second defines it as the number of new outbound call setups allowed in one second, and the default is one per trunk per region. Any increase carries a monthly fee. The console allows a setting of up to five, though a new account without an approved Business Profile cannot make that change itself. Rates above five go through Twilio's sales team.

Suppose an appointment-reminder system tries to start 60 calls at the same instant on a trunk held to one call per second. Twilio rejects attempts beyond the limit, so the system has to pace itself to one new call a second and needs about a minute to get all 60 under way. The trunk could hold all 60 calls at once, but it will not start them at once.

Automated dialing is the obvious way to reach this limit, and a large team dialing by hand can reach it too.

A provider's allowance is only usable if everything on your side can keep up. A PBX, a session border controller and a gateway each have a maximum number of calls they handle at once. For some it is a license and for others it is a property of the hardware. Our answer on the equipment SIP trunking needs covers which of those devices you would have.

The data connection is the other ceiling. Twilio's SIP trunking requirements give a planning figure of 100 kbps for each simultaneous call on the G.711 codec, with room for overhead included. By that allowance, the 27-call example needs 2.7 Mbps available for voice at the peak. That is Twilio's planning number, and other codecs and network setups produce different ones. We go through them in the connection and bandwidth SIP trunking needs and how much bandwidth a VoIP call uses.

The number you get is the smallest of these limits. Suppose the plan allows 27 calls and the gateway in front of your PBX handles 20. The usable ceiling is 20 calls, and seven channels you are paying for go unused. Sizing the plan, the equipment and the connection for the same busy hour avoids that.