What is SIP trunking?

SIP trunking is the service that connects your business phone system to the public telephone network over the internet using the SIP protocol. It replaces the physical phone lines (T1, PRI, copper) that used to connect PBX systems to the outside world. With a SIP trunk, a phone call originates on the internet and terminates on the telephone network — or vice versa — with no physical phone line in between.

SIP stands for Session Initiation Protocol. It's the signaling standard used to set up, modify, and tear down voice calls over the internet. A SIP trunk is just a SIP-based connection between your phone system and a service provider that bridges to the phone network.

What SIP trunking replaces

Historically, a business PBX connected to the phone network through physical lines:

• Analog lines — one pair of copper wires per phone call. Simple but expensive at scale.

• T1 lines — 24 concurrent calls per T1, delivered as digital signal over dedicated circuits

• PRI (Primary Rate Interface) — an ISDN-based digital line supporting 23 concurrent calls plus signaling. The dominant business standard through the 2000s.

SIP trunks replaced all of this with a software connection over your existing internet. The benefits: lower cost, faster provisioning, flexible capacity, and richer features.

How SIP trunking works

• Your PBX connects to a SIP trunk provider over the internet using the SIP protocol

• When a call comes in from the phone network, the provider converts it to a SIP call and delivers it to your PBX

• When you place an outbound call, your PBX sends a SIP request to the provider, which converts it back to the phone network

• You pay per concurrent channel, per minute, or per user — depending on the provider's pricing model

SIP trunking vs. hosted PBX

The distinction often confuses people.

• Hosted PBX (UCaaS) — the phone system itself lives in the cloud (RingCentral, Zoom Phone, Nextiva, etc.). You pay per user, get phones that register to the cloud, and don't worry about how calls get to the phone network.

• SIP trunking — you have your own PBX (on-premise or cloud-based), and SIP trunking is the connection between it and the phone network.

Hosted PBX providers use SIP trunks internally, but you don't see them. SIP trunking as a separate service is usually purchased by businesses running their own PBX (Asterisk, FreePBX, Cisco CUCM, 3CX, Avaya IP Office, etc.).

When SIP trunking matters for AI voice agents

For most businesses, deploying an AI voice agent doesn't require thinking about SIP trunking — you just forward your existing number. But advanced deployments route calls directly from the business PBX to the AI over SIP, which:

• Reduces latency by cutting out the extra hop through the phone network

• Preserves richer call metadata (the original caller's number, caller ID name, the internal destination)

• Avoids per-minute forwarding fees charged by some carriers

• Supports advanced features like conditional routing based on internal PBX state

If your business runs Cisco CUCM, Avaya, or another self-managed PBX and you want SIP-direct integration with our AI, we support it — see our integrations page.

Related concepts

• VoIP — the umbrella technology SIP is part of

• PBX — the phone system SIP trunks connect

• Call forwarding — the simpler alternative to SIP integration

• Number porting — moving numbers between providers

See it in action

At 365agents we support both simple call-forwarding deployments (for most SMBs) and SIP-direct integration (for enterprises running their own PBX). See the integrations page for the supported PBX platforms and integration paths.