What is a SIP trunk?

A SIP trunk is what replaced racks of physical phone lines in modern offices: one internet connection, carrying many simultaneous calls, in place of one wire per call.

A virtual replacement for physical phone lines

A traditional office phone system connects to the outside world through physical phone lines, one wire per simultaneous call. A SIP trunk replaces all of those wires with a single internet connection to a SIP provider, who handles routing your calls onto the wider phone network on your behalf.

The name comes directly from SIP, the protocol that carries the call setup, and "trunk" because in traditional telephony a trunk line is any connection carrying many calls between two switching points at once.

Why businesses switched

A SIP trunk is not billed per physical line, so a business can scale from ten simultaneous calls to a hundred without running new cables. It is also usually far cheaper than the equivalent number of traditional phone lines, and it lets a single SIP trunk carry calls for offices in completely different cities.

What actually flows over it

A SIP trunk carries the same two things every internet call does: SIP signaling to set up and tear down each call, and the media stream itself once the call connects. Your office PBX talks SIP to the provider's trunk, and the provider is responsible for actually reaching whatever number you dialed, wherever it lives.