SIP trunking

SIP stands for Session Initiation Protocol. The system on the business’s side can be an office phone system, contact-centre software or an AI voice agent. One trunk carries many calls at once, and the business’s phone numbers are routed over it.

How it works

SIP is the signalling protocol the IETF published as RFC 3261 (2002) for “creating, modifying, and terminating sessions”, internet telephone calls among them. On a SIP trunk:

• Signalling (SIP messages) sets up a call, rings the other side and ends it.
• Media, the audio itself, flows separately once the call connects, usually as RTP packets.
• Numbers are attached to the trunk. Calls to a DID number arrive on it; outbound calls leave through it with a caller ID the carrier permits.
• Channels set how many calls can run at the same time, and are changed in configuration rather than by installing hardware.

The carrier connects the trunk to India’s mobile and fixed networks, so a customer on any operator can call in or be called.

Why it matters in India

The older route to many lines is a PRI: a physical E1 circuit from the operator that carries up to 30 calls and ends in a box in the office. A SIP trunk has no box. Capacity grows by configuration, and the system answering the calls can run in a data centre.

For an AI voice agent, the trunk is the bridge between the software and the phone network. Its quality is audible: packet loss and jitter sound like choppy audio, and every extra network hop adds to voice latency.

Trunks are also where regulators act. TRAI’s 2025 amendment to TCCCPR (PDF), opens in a new tab provides that a sender found guilty of repeated violations loses all its telecom resources, including PRI and SIP trunks, across operators for a year. In the Third Amendment of September 2026 (PDF), opens in a new tab, TRAI reported that 15,736 of 20,591 disconnections for such violations in April 2026 were SIP or PRI connections used for automated calls.

Example

Say a Mumbai brokerage publishes one 022 number. Over a SIP trunk, that number can ring the office phones during working hours and an AI voice agent in the cloud after hours, with nothing changing for the caller. Adding a second project’s number means adding a DID to the same trunk, not laying a new line.

Common confusions

• SIP trunk vs virtual number. The trunk is the connection; a number is an address routed over it. One trunk can carry many numbers.
• SIP trunk vs calling apps. App-to-app calls, such as WhatsApp calls, stay between app users. A SIP trunk reaches ordinary phone numbers on any network.
• SIP trunk vs registration. A trunk carries calls. It does not register a business as a telemarketer or allot a 140-series number for promotional calls; those come through the operator under TCCCPR and DLT.
• SIP vs VoIP. VoIP is the general idea of voice over IP networks. SIP is one protocol for setting up those calls, and a SIP trunk is a commercial service built on it.