What is a SIP trunk?
A SIP trunk is a virtual connection that carries voice calls over an IP network instead of a physical phone line. SIP, the Session Initiation Protocol, sets up and tears down the calls, while the audio flows as IP packets. One trunk can carry many simultaneous calls, and capacity is defined in software rather than copper.
SIP versus ISDN
Traditional ISDN delivers calls in fixed blocks of channels over dedicated lines. SIP trunking replaces that with flexible, IP-based capacity that scales on demand.
- Elastic capacity: add or remove channels in software, not weeks.
- Lower cost: no dedicated physical lines to rent per site.
- Resilience: trunks can re-register to a second data centre on failure.
- Reach: numbers from many countries terminate on one connection.
What you need to connect
Any standards-compliant IP-PBX, session border controller or unified communications platform can connect, including Asterisk, FreeSWITCH, 3CX, Cisco, Avaya and Microsoft Teams Direct Routing. Authentication is by IP address or digest, with optional TLS and SRTP encryption.
What to look for
- Geo-redundant points of presence for continuity.
- Fast number and trunk provisioning.
- Per-trunk spend caps and fraud controls.
- Transparent, usage-based billing with real-time CDRs.