SIP trunking is a way to connect a business phone system (PBX) to the public telephone network over an IP connection instead of traditional phone lines. A SIP trunk is a virtual set of phone lines delivered by a provider, and each simultaneous call uses one channel.
How SIP trunking works
Your PBX talks to the provider using the Session Initiation Protocol (SIP). When someone dials out, the PBX asks the provider to place the call, and the provider connects it to the regular phone network. Incoming calls take the reverse path. The voice itself travels as IP packets over your internet or a private link.
SIP trunking vs PRI and analog lines
| SIP trunk | PRI / ISDN | Analog lines | |
|---|---|---|---|
| Connection | IP network | Dedicated digital circuit | Copper pair per line |
| Scaling | Add or remove channels | Fixed blocks of channels | One line at a time |
| Needs | Bandwidth, PBX with SIP | PRI card and circuit | Line per call |
| Geographic flexibility | High | Low | Low |
What are channels?
A channel is one simultaneous call. If you rarely have more than 12 calls at once, 12 channels may be enough even if you have 50 employees. To size this properly, use traffic data and our Erlang calculator, which estimates how many lines you need for a target blocking rate.
Pricing models you will see
- Per channel: a flat monthly fee for each concurrent call path
- Metered: you pay per minute used
- Bundles: a channel count with included minutes
Prices vary by provider and region, so compare current quotes rather than relying on averages.
What you need before you switch
- A PBX or IP-PBX that supports SIP trunks
- Enough upload and download bandwidth for peak calls (see the bandwidth calculator)
- A properly configured firewall or session border controller
- A plan for number porting and emergency calling
- A backup path such as failover routing to a mobile number
If you do not have a PBX yet, a cloud phone system may be simpler than SIP trunking, because the provider hosts the PBX for you.
Frequently asked questions
How many SIP trunk channels do I need?
Count your peak simultaneous calls, then add a margin. An Erlang B calculation based on call volume and call length gives a more reliable number.
Can SIP trunking replace my existing phone lines?
In most cases yes, provided your PBX supports SIP and your network has enough bandwidth.
Is SIP trunking the same as VoIP?
SIP trunking is one way to deliver VoIP. It specifically connects a PBX to the phone network.
