SIP trunk

A SIP trunk is a virtual connection that links a business phone system or cloud call platform to a carrier's network over the internet instead of a physical cable. Inbound and outbound calls both travel over it, and the business's numbers are assigned to it. A single SIP trunk can carry several calls at once; its number of channels decides how many can run at the same time.

How it works

There are two common ways to connect. With a registration-based trunk, the phone system or platform registers with the carrier using a username and password, much as a desk phone would, and calls arrive through that registration. With an IP-based trunk, the carrier and the platform recognise each other's fixed IP addresses and no registration is needed. Either way, signalling uses SIP and the audio travels over RTP. How many concurrent calls your trunk can carry depends on your agreement with the carrier.

Why it matters

A SIP trunk is the most common way to move to a new calling setup without changing your business number. You keep your contract with your current carrier and route calls to a cloud phone system or an AI voice agent. If the trunk has too few channels, callers hear a busy tone at peak times, so plan its capacity around your call volume.