SIP Trunking
Complete guide to SIP lines and channels
Learn exactly what a SIP Trunk channel is, how it works, and everything else you'll need to know to scale your call capabilities.
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.
"A SIP trunk channel is the digital equivalent of a single phone line—it represents one active call path that can carry either an inbound or outbound call. When that call ends, the channel becomes available for the next one."
— DIDlogic, Enterprise Telecommunications Provider
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.
According to TechTarget, a SIP trunk is a virtual phone line that connects a private branch exchange (PBX) to the PSTN and may consist of 20 or more SIP lines or channels.
How do SIP trunk 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.
For a deeper dive into the technical components, check out our SIP trunking architecture guide or explore how to set up a SIP trunk.
How are SIP channels billed?
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.
SIP trunk vs PRI vs VoIP comparison
Understanding the differences between these technologies helps you choose the right solution for your business. Here's a comprehensive comparison:
| Feature | SIP Trunk | PRI | VoIP (General) |
|---|---|---|---|
| Connection Type | Virtual (Internet-based) | Physical copper wires | Internet-based |
| Channels per Line | Unlimited (bandwidth-dependent) | 23 channels per T1 | Varies by provider |
| Scalability | Add/remove channels instantly via portal or API | Requires physical installation (2-3 weeks) | Varies; typically flexible |
| Infrastructure | Existing internet connection | Dedicated copper circuits | Internet connection |
| Cost Model | Per-channel, pay-as-you-go | Fixed bundles of 23 channels | Per-user or per-minute |
| Media Types | Voice, video, messaging, fax | Voice and data only | Primarily voice |
| Setup Time | Minutes to hours | Weeks | Hours to days |
| Geographic Flexibility | High (use from anywhere) | Low (location-bound) | High |
| Redundancy | Built-in failover options | Limited; physical line dependent | Provider-dependent |
| Typical Savings | 25-60% vs traditional lines | Baseline cost | 20-50% vs PRI |
Sources: Spectrum Business, Nextiva, AWS
According to a Nextiva analysis, SIP trunking is rapidly becoming the preferred choice for businesses seeking a future-proof, scalable, and cost-effective telephony solution. PRI remains viable for organizations with stable call volumes and existing infrastructure investments, but its limitations in scalability make it less versatile for modern business needs.
How many channels are 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.
Channels to concurrent call capacity
Use this table to estimate how many SIP trunk channels your organization needs based on employee count and business type:
| Number of Employees | Employee-to-Channel Ratio | Recommended Channels | Best For |
|---|---|---|---|
| 2-5 | 1:1 | 2-5 | Small offices where most staff use phones simultaneously |
| 10-20 | 3:1 to 4:1 | 3-7 | General business offices |
| 25-50 | 3:1 to 4:1 | 8-17 | Mid-size companies with moderate call volume |
| 50-100 | 4:1 to 5:1 | 10-25 | Corporate offices, professional services |
| 100-250 | 4:1 to 6:1 | 17-63 | Large enterprises with mixed departments |
| Call Centers | 1:1 to 2:1 | Equal to or half of agents | High-volume contact centers |
Note: Add 20-25% buffer capacity to handle unexpected spikes. Sources: SIP.US, Net2Phone, Flowroute
"You don't need a one-to-one match between employees and SIP channels. Most businesses find that one channel for every three to four employees is a reasonable baseline. That said, this ratio can shift depending on your communication habits."
— SIP.US, SIP Trunking Capacity Guide
What is the maximum number of SIP channels?
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 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.
Getting started with SIP trunking
Ready to deploy SIP trunking for your business? Here are resources to help you get started:
Developer resources:
- SIP Trunking Quickstart Guide — Step-by-step instructions for setting up your first SIP connection
- SIP Trunking Documentation — Complete API reference and configuration guides
Support resources:
- How to Configure a SIP Trunk — Detailed walkthrough for SIP trunk setup
- SIP Trunking Configuration Guides — Guides for connecting Telnyx with popular PBX systems including Cisco, Avaya, 3CX, Asterisk, and more
Solutions for your industry:
- Contact Center Solutions — Scale voice communications for high-volume customer support
- Healthcare Communications — HIPAA-compliant voice solutions for patient engagement
Related products:
- Voice API — Build programmable voice applications with advanced call control
- Global Numbers — Provision local, toll-free, and international numbers instantly
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.
Frequently Asked Questions
How many SIP channels do I need for my business? Most businesses need one SIP channel for every 3-4 employees, though this varies based on call volume. Call centers typically need a 1:1 or 2:1 ratio of agents to channels. Review your concurrent call data or contact your provider for a personalized assessment.
What's the difference between a SIP trunk and a SIP channel? A SIP trunk is the virtual connection between your phone system and your service provider. SIP channels are the individual call paths within that trunk. One SIP trunk can support unlimited channels, limited only by your bandwidth.
Can I keep my existing phone numbers when switching to SIP trunking? Yes. Number porting allows you to transfer your existing phone numbers to a SIP trunking provider. The FCC guarantees your right to port numbers when switching providers.
Additional sources: IR.com Fundamentals of SIP Trunking, Bandwidth SIP Trunks Guide, Business.com SIP Trunking Overview, Telavox SIP Trunking Explained
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