What is SIP trunk?

In a traditional phone system, voice calls travel over copper or ISDN lines provisioned by a phone company — physical trunks. SIP trunking does the same job over the internet using the SIP protocol (a signaling protocol developed in the late 1990s) to set up calls and RTP (Real-time Transport Protocol) to carry the actual voice audio.

A business with a SIP trunk gets a block of phone numbers that ring through to its on-premise PBX, cloud phone system, or AI voice agent. Outbound calls reverse the flow: the PBX hands off to the SIP trunk provider, who completes the call to the destination over the public phone network.

AI voice agents typically receive calls through a SIP trunk. The trunk provider (Twilio, Bandwidth, Telnyx, SignalWire, and others) hands the call to the voice agent over SIP, the agent runs its STT-LLM-TTS loop, and the audio flows back over the same connection to the caller.

SIP integration is what lets an AI voice agent answer calls to an existing business phone number without porting numbers or changing telephony providers. Most modern voice agent platforms (LiveKit, Vapi, Retell, Synthflow) provide SIP connectors out of the box.

Law firms typically have an existing phone number that callers, referrals, and Google Business Profile entries already point to. Porting that number is risky and slow. SIP trunking lets the firm keep the number and the existing carrier relationship while routing inbound calls to the AI voice agent — and routing outbound transfers back to attorneys via the same trunk.

For multi-office firms (Vasquez Law Firm runs 4), SIP trunks per office allow per-location call routing rules without abandoning existing telephony.