In short. A SIP trunk is a virtual phone line that connects your company’s phone system to the public telephone network over the internet. It replaces physical ISDN or analogue lines, allows several simultaneous calls over the same connection, and can reduce costs, depending on current traffic, the number of channels and the rates agreed, while keeping your existing numbers.
What is a SIP trunk
SIP (Session Initiation Protocol) is the standard protocol that sets up and manages voice calls over the internet. A “trunk” is, traditionally, a bundle of phone lines. A SIP trunk combines the two: a logical channel through which your phone system sends and receives calls to and from the public network, without a dedicated physical line for each simultaneous call.
Instead of paying for a limited number of physical lines, you get a single trunk carrying several simultaneous calls, sized to your actual needs.
How it works
- Your phone system (Asterisk, FreePBX, 3CX, Blureon PBX and similar) authenticates with the operator over SIP.
- Outbound calls leave through the trunk to the public network, on routes chosen by the operator for the call destination.
- Inbound calls to your numbers are delivered to your phone system through the same trunk.
- Media (voice) travels encrypted (SRTP) or in standard form (RTP), using codecs such as Opus or G.711.
SIP trunk vs traditional lines
| Feature | ISDN/analogue line | SIP trunk |
|---|---|---|
| Capacity | Fixed, with less flexible upgrades | Scalable on demand |
| Cost | Fixed cost model, regardless of traffic | Cost model adjusted to the number of channels and traffic |
| Numbers from other cities | Difficult | Straightforward |
| Failover | Limited | Automatic |
| Modern features | Few | Many (CRM, reporting, AI) |
What to check when choosing a provider
- Who handles routing — ask who actually handles routing, support and incident response.
- Transparent rates — a clear price per destination, with no minimum usage and no hidden costs.
- Failover — what happens if the internet goes down at your site? Check the call forwarding options and the conditions under which they can be enabled.
- Portability — being able to bring your existing numbers in, and take them with you if you leave.
- Technical support — talking to engineers, not a call centre.
Conclusion
For many companies, a SIP trunk is a suitable option for connecting their phone system to the public network, once you have checked the infrastructure and your call requirements. Before choosing, check the number of simultaneous calls you need, your phone system’s compatibility, the redundancy options and the pricing model. If you would like to see how this would work for your phone system, request a quote.