What is a SIP trunk channel?

The word is borrowed from older phone service, where capacity came as lines and each line carried one call. SIP.US's pricing page makes the comparison directly. It suggests thinking of SIP channels as the equivalent of old-fashioned telephone lines, and says you need one for every concurrent incoming or outgoing call. The same page calls the trunk "the container for your SIP channels." The trunk is the connection between your phone system and the public telephone network. The channels are how many calls that connection is allowed to carry.

A line was a circuit that carried one call. A channel is a count of the calls a provider permits. A call on a SIP trunk is a set of setup messages and a stream of audio packets crossing an IP network, as our answer on how SIP trunking works shows. The protocols involved do not reserve capacity for it. RFC 3550, which defines the audio transport, says RTP "does not address resource reservation." So the channel count is a number in the provider's configuration, and raising it need not mean running a new circuit. The calls it permits still depend on equipment and a connection that can handle them.

That is why a channel count can move with demand. Atlantech Online's April 2023 post on its SIP trunks describes channels as capacity that can be added. Its example is a local government customer that adds capacity on its trunks during weather events such as snowstorms and tropical storms. The provider's limit is the part that flexes this way. The extra calls still need room on the customer's data connection and on the equipment at its end of the trunk.

Suppose an office has a trunk with eight channels, shared between incoming and outgoing calls. At mid-morning five customers have called in and three employees have called out. All eight channels are in use. A ninth call in either direction has no channel to take. What the caller hears then depends on how the provider and the phone system are set up. When any one of the eight calls ends, its channel is free for the next call.

Idle things use no channels. A phone sitting on a desk uses none, and neither does a number nobody is calling. A call between two extensions that the PBX routes locally stays inside the PBX and never reaches the trunk.

One conversation can use more than one channel. Suppose a customer's call arrives on the trunk and the PBX forwards it to an employee's mobile phone. If the PBX places that second call out through the same trunk, both calls stay up for the length of the conversation, and it holds two channels. Phone systems handle forwarding in different ways, so this does not happen in every setup. The safe way to count is by calls passing through the trunk. Conversations can undercount them.

Whether incoming and outgoing calls share one pool is a term of the plan. SIP.US counts a call in either direction against the same channels. Another plan might count the two directions separately, and that changes how many of each you can have.

A phone number is an address, and it holds no capacity of its own. SIP.US says any quantity of numbers can be mapped to a trunk, and it prices them separately from channels. A local number in the lower 48 states is $1.00 a month with a $1.00 setup fee. A channel is $24.95 a month on the month-to-month plan.

Suppose an office wants 20 of those local numbers and eight channels on that plan. The channels come to $199.60 a month and the numbers to $20.00, so the total is $219.60 a month before taxes and fees, plus $20.00 once for setup. Any of the 20 numbers can ring, but only eight calls can be in progress at once across all of them.

Employees are not channels either. A channel is used by a call, so an employee who is not on an outside call uses none. The count that matters is the number of outside calls in progress at the busiest moment. Our answer on how many simultaneous calls you need shows how to work it out.

Calls per second measures something else again. Twilio's page on the subject defines it as how many new outbound call setups are allowed in one second, and sets a default of one per trunk per region. That limit governs how quickly calls can start. It says nothing about how many can be in progress. A trunk can have room for many calls and still reject a burst of them that all begin in the same second.

A channel is permission for a call, and the data connection is what carries it. Twilio's SIP trunking requirements give a planning figure of 100 kbps for each simultaneous call on the G.711 codec, with overhead included. By that allowance, eight calls in progress need 800 kbps available for voice. A provider's channel count does not check that your connection has that room. Our answer on the connection and bandwidth SIP trunking needs covers the sizing.

The channel is a commercial unit, and providers are free to use it or not. SIP.US prices its trunks by the channel. Twilio's limits page for Elastic SIP Trunking lists unlimited concurrent calls and no channel count. It attaches two conditions: the figure depends on carrier support, and new accounts without an approved Business Profile have limited concurrent calls. Atlantech's business phone page describes its pricing as tailored and publishes no per-channel rate, so on a trunk like that the capacity is something to ask for in the quote.