Definition
SIP trunking is a way to connect a phone system, or a voice agent, to the public phone network over an IP connection to a carrier. The carrier supplies phone numbers and a limit on how many calls can run at once.
Read the full guide: What is SIP trunking? How it works for voice AI
This is the short definition. The SIP trunking guide covers how it works for voice AI.
How SIP trunking works
A trunk carries two things. Signaling uses the Session Initiation Protocol, defined in RFC 3261, and sets up and ends each call with messages such as INVITE and BYE. The audio travels separately as RTP packets, defined in RFC 3550, usually on a different set of ports. For an inbound call, the carrier owns the dialed number, looks up your trunk and sends an INVITE to the address you configured. Outbound calls run the same steps in reverse.
The trunk itself is a configured, authenticated connection to one carrier. It has a concurrency limit, often counted in channels, and it can hold many phone numbers. Calls leave the trunk and reach ordinary phones through the public switched telephone network.
Why it matters for voice agents
A trunk lets a voice agent answer calls on numbers and carrier contracts you already have, instead of numbers issued by the agent platform. Two limits come with it. The trunk’s channel count and the platform’s own cap both bound concurrent calls, and the lower one wins. The audio on the trunk is usually narrowband telephony audio, which affects recognition accuracy.
Cartesia Managed Agents accept inbound and outbound calls over SIP trunks using TCP or TLS, with optional SRTP encryption for the audio. A call from a browser or app does not cross the phone network at all and can use WebRTC instead.
For setup steps, authentication and troubleshooting, read the full guide.