SIP Trunking

SIP trunking is the delivery of voice calls over an IP network using SIP (Session Initiation Protocol), replacing the physical phone lines of the legacy telephone era with a virtual connection between two systems. A “trunk” is that standing connection: between your company and a carrier, between a PBX and the PSTN, or — in the voice AI context — between your telecom infrastructure and an AI platform’s media servers.

SIP itself handles the signaling: setting up, modifying, and tearing down calls, negotiating capabilities, and carrying metadata like the dialed number (DNIS) and caller ID. The actual audio travels separately as RTP media streams, encoded with codecs negotiated during setup — G.711 is the universal PSTN baseline; Opus offers better quality where both ends support it. Trunks authenticate by IP allowlisting (each side trusts the other’s known signaling addresses) or SIP registration (credentials), and production deployments should use TLS for signaling and SRTP for media encryption.

Why it matters for voice agents

SIP trunking is the connective tissue of every serious phone-facing AI deployment. It’s how BYOC works — your existing carrier delivers calls to the AI platform over a trunk, so you keep your rates, DID numbers, and compliance posture. It’s also how voice agents integrate with existing contact-center infrastructure: a trunk between your PBX or CCaaS and the AI platform lets agents receive escalations from and perform warm transfers back to human queues.

The engineering details leak directly into conversation quality. Every transcoding hop between codecs adds latency and degrades the audio that speech recognition depends on. Trunk geography matters too: if the platform’s SIP ingress is a continent away from your carrier’s breakout, you pay the round-trip on every packet — a fixed tax on time to first audio. And operational basics — DTMF handling (RFC 2833), SIP REFER vs. bridged transfers, failover when an endpoint is unreachable — are worth testing before launch, not after.

Related terms

• BYOC (Bring Your Own Carrier) — the deployment model built on SIP trunks
• DID Number — the numbers routed across the trunk
• Time to First Audio (TTFA) — the latency metric trunk geography affects