Insights & Resources4 min read

What is a SIP Trunk Channel? Complete Guide to SIP Lines

Learn exactly what a SIP Trunk channel is, how it works, and everything else you'll need to know to scale your call capabilities.

Brian Segal
Copper wire SIP Trunk channel
If you’ve read anything about SIP trunking, you’ve probably heard about SIP trunk channels. Most billing methods include something about SIP trunk channels, but what actually is a SIP trunk channel?
A better understanding of SIP trunk channels will help clarify how your SIP trunking service works and how you pay for your SIP trunk connections.

What is a SIP Trunk Channel?

A SIP trunk channel is a line for a single call on your SIP trunk. If you want to make multiple simultaneous calls, you’ll need a SIP trunk channel for each of those concurrent calls. A SIP trunk channel is comparable to a single phone wire in a PRI trunk.
However, adding SIP trunk channels does not require adding more physical wiring. New channels are added virtually. The SIP trunk uses your internet connection to connect to the networks needed to connect calls to the receiving phones.
What about SIP lines? Is there a difference between a line and a channel on a SIP trunk?

SIP Channels vs SIP Lines

The short answer is that SIP channels and SIP trunk lines are the same thing. Some people prefer the term ‘SIP trunk line,’ because the terminology is more consistent with traditional telephone lingo. Each connection on the legacy PSTN was called a “line” and referred to the copper wire connection between two phones. But the terms “SIP trunk channel” and “SIP trunk line” are interchangeable.

How SIP Channels Work

Traditionally, the term “trunk” referred to a thick bundle of phone wires that held 23 phone lines. This pack of wires is a PRI trunk. A SIP trunk is a modern version of this thick bundle of phone lines.
Each SIP channel is like one of those phone lines in the bigger pack of wires. A SIP channel is capable of connecting a single phone call. And one of the biggest benefits of a SIP trunk is that a SIP trunk can hold far more channels than the number of wires in a traditional phone line trunk.
When you make a phone call through your SIP trunk, your phone opens a connection on the trunk (also called a session, hence the acronym for “Session Initiation Protocol”). That call connection takes up one SIP trunk channel. And that channel creates a connection to either the public internet or your telecom carrier’s network to connect the call. If someone else makes a concurrent call, it simply opens up another connection, using a different channel on your SIP trunk.
The thing that makes this different from a traditional phone line trunk is that both of these calls get transmitted over the same wire: your internet connection, which is usually an ethernet cable or fiber optic line. So it requires very little additional cable infrastructure to get as many SIP trunk channels as you need. But, like traditional phone lines, the number of SIP trunk channels that you use will affect your telephone bill.

How They’re Billed

Most SIP trunking providers sell channels in batches. Often, the price per channel goes down as you purchase more channels.
For instance, SIP trunk channels on the Telnyx network are billed at $12 per month for your first 10 channels. The next 40 channels you purchase are billed at $10 per month. And the next 200 channels cost just $9 per month.
In contrast, you can only get traditional phone lines in bundles of 23 lines. Each pack of 23 lines is usually billed at the same rate—no volume discounts. And you must pay to have a new pack of 23 wires installed if you need to make more than 23 simultaneous calls. Obviously, traditional phone lines work, but the pricing and infrastructure is much less flexible and scalable.

How Many Channels in a SIP Trunk?

Technically, there is no set number of channels in a SIP trunk. The number of channels on your SIP trunk will expand and contract as you connect and disconnect calls. So, if you have 20 calls going at once, your SIP trunk will have 20 channels to accommodate these calls.
Then, if you disconnected all these calls, your SIP trunk would contract to zero channels. Automatically adding and disconnecting SIP trunking channels like this is known as “elastic SIP trunking.”
In short, your SIP trunk will have as many channels as you need, based on the number of simultaneous calls you want to make. Just keep in mind that it’s best to purchase the SIP trunking channels from your telecom provider before you need them.

What is the Maximum Number of Channels?

There really is no maximum number of channels. Your SIP trunk can handle a potential unlimited number of SIP trunk channels.
However, each channel will require a certain amount of internet bandwidth, depending on factors such as the VoIP codec you’re using. Certain codecs require less bandwidth per call.
But your internet connection is the limiting factor for how many SIP trunk channels you can have. A very rough estimate is that you’ll need about 115kbps of internet bandwidth per SIP trunking channel. So, if you have a 1.5mbps internet connection, your maximum number of channels would be about 12.
To sum up, your SIP trunk is capable of handling as many channels as you need. The SIP trunk itself sets no maximum number of channels.
If you need to know the maximum number of channels you can run, look at your internet connection, since your internet bandwidth is what limits how many SIP channels you can have connected at once. You can use this VoIP bandwidth calculator if you want to precisely calculate your bandwidth needs.
Once you know how many channels your internet connection can handle, get in touch with a Telnyx expert to explore SIP Trunk pricing.
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