SIP Trunk
A SIP trunk connects a PBX to the public telephone network over the internet, replacing traditional phone lines. Learn how it carries calls and how it is configured.
Published 8 November 2026 · 1 min read · By SpaGreen Creative
Short answer
A SIP trunk is a service from a carrier that connects a PBX to the public telephone network over IP using SIP, carrying multiple concurrent calls in place of physical phone lines.
A PBX on its own can only connect its extensions to each other. To reach ordinary phone numbers it needs a link to the public telephone network. Historically that link was a bundle of analogue lines or a digital circuit such as ISDN. A SIP trunk replaces those with a SIP connection over the internet to a carrier, who then hands calls to and from the public network.
The word trunk comes from traditional telephony, where it meant a line between two switches rather than a line to a single phone. A SIP trunk behaves the same way: it carries many simultaneous calls, limited by the plan rather than by physical wires. The carrier assigns one or more public numbers, known as DIDs, to the trunk and routes inbound calls to the PBX.
Configuration on the PBX side involves the carrier's SIP server address, a method of authentication, which is either a username and password with registration or an IP address allow-list, the allowed codecs and the number format the carrier expects. Asterisk-based systems configure this as a trunk in FreePBX or Issabel, or as a PJSIP endpoint and registration in plain Asterisk.
A softphone does not connect to a trunk directly. It registers with the PBX, and the PBX uses the trunk for external calls.
Sources and further reading
- RFC 3261: SIP: Session Initiation Protocolwww.rfc-editor.org
- Asterisk documentationdocs.asterisk.org
This article is published by SpaGreen Creative, the maker of SIPdia. Product names mentioned belong to their owners. Where it describes other software, treat the steps as general guidance and check the official documentation linked above.
