What is a SIP trunk?

Phone calls used to need physical lines. A business that wanted thirty calls at once rented a PRI, a fat copper or fibre circuit from the telephone exchange. A SIP trunk does the same job over an internet connection, and adding capacity is often just a change to your plan.

SIP, the Session Initiation Protocol, is the signalling language. It sets up a call, rings the other end, changes the call if needed, and hangs up. The audio itself travels separately, as RTP packets, in a format both ends agree on during setup. On phone networks that’s usually G.711 μ-law or A-law at 8 kHz.

When people talk about a trunk, they usually mean three things bundled together. There’s the connection itself, the phone numbers that ring on it, and the number of channels: how many calls it can carry at the same moment.

For a voice agent, the trunk is the road between the AI and the phone network. Calls arrive on it from callers, or leave on it for a campaign. Whoever owns the trunk holds the carrier relationship and the per-minute bill, and shares the responsibility under TRAI’s rules for the calls it carries.

That responsibility is real. TRAI’s 2026 amendment makes bulk callers declare automated calls and the numbers they’ll use, and it lets operators bar outgoing service on a sender’s SIP and PRI lines after repeated spam flags. In April 2026, TRAI reports, about three in four connections disconnected over these violations were SIP or PRI lines used for automated calls.

So before you point an AI agent at a trunk, check four things. Is the sender registered? Which codecs does the trunk offer? How many channels do you actually need at peak? And who answers when a call fails at three in the morning?