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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.
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?
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.
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.
Understanding the core components of SIP trunk channels helps you make informed decisions about your telecommunications infrastructure:
| Component | Description | Function |
|---|---|---|
| SIP channel | Virtual line for a single concurrent call | Enables one inbound or outbound call at a time |
| SIP trunk | Virtual connection bundling multiple channels | Routes voice traffic over IP networks to PSTN |
| Session Border Controller (SBC) | Security and protocol normalization device | Protects network boundaries and manages NAT |
| Voice codec | Audio compression/decompression algorithm | Determines bandwidth usage and call quality |
Most SIP trunk 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.
When evaluating your business communication options, understanding how SIP trunking compares to alternative technologies helps you make the right choice for your organization:
| Feature | SIP Trunking | PRI Lines | POTS Lines | Hosted VoIP |
|---|---|---|---|---|
| Scalability | Unlimited, instant | 23 channels per circuit | 1 call per line | Provider-dependent |
| Cost savings | 30-50% vs PRI | Baseline | Highest per line | Variable |
| Infrastructure | Internet connection | Dedicated circuit | Copper wires | Internet + cloud |
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.
There really is no maximum number of SIP 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 80-115kbps of internet bandwidth per SIP trunking channel, depending on your codec. So, if you have a 1.5mbps internet connection, your maximum number of channels would be about 12-18.
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.
Different business scenarios benefit from SIP trunk channels in unique ways. Here's how various use cases align with specific advantages:
| Use Case | Primary Benefit | Channel Recommendation |
|---|---|---|
| Contact center | Elastic scaling during peak periods; 40-60% concurrency ratio | 1 channel per 2-3 agents |
| Multi-site enterprise | Centralized management; unified communications across locations | 1 channel per 3-4 employees |
| Small business | Cost reduction of 30-50% vs traditional lines | 1 channel per 4-5 employees |
| Healthcare provider | HIPAA-compliant communications; reliable patient outreach | 1 channel per 2-3 staff |
Industry experts emphasize the strategic importance of SIP trunking for modern business communications:
"The big reason to move to SIP trunks is simple: You can still save up to 50% over your PRI bill. As Ethernet access services continue to decline in price, the underlying access costs on SIP trunks are also cheaper—making the ROI case even stronger for enterprises modernizing their voice infrastructure."
— Sorell Slaymaker, Research Director, Gartner
"Most organizations can reduce channel counts by 40-60% compared to legacy systems without impacting performance. The key is proper traffic analysis and right-sizing your deployment. Overprovisioning channels increases monthly expenses unnecessarily, while underprovisioning risks call quality issues and customer experience degradation."
— Irwin Lazar, President and Principal Analyst, Metrigy
Ready to implement SIP trunking for your organization? Here are some resources to help you get started:
Contact our team of experts to set up SIP trunk channels for your organization. Or check out our SIP trunk pricing to learn how much you could save.
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| Business phone system using IP protocols |
| Manages internal call routing and extensions |
| DID (Direct Inward Dialing) | Phone numbers assigned to your SIP trunk | Allows direct external dialing to specific users |
| Setup time | Minutes to hours | Weeks | Days to weeks | Hours to days |
| PBX control | Full ownership | Full ownership | Limited | Provider-managed |
| Long-distance rates | Low (IP-based) | Higher | Highest | Included/bundled |
| Low-latency connections; seamless PSTN integration |
| Based on concurrent AI agent sessions |
| Remote workforce | Location independence; consistent call quality anywhere | 1 channel per 5-6 remote workers |
