What is SIP trunking?

SIP trunking is a method of connecting a phone system or application to the public telephone network (PSTN) over an IP connection using the Session Initiation Protocol (SIP). It replaces physical phone lines with virtual channels, each carrying one concurrent call, and supports both inbound and outbound calling as data.

How it works

SIP is the signaling protocol that sets up, manages, and tears down voice sessions, while the media (audio) flows over RTP. A SIP trunk is the logical link between a customer’s PBX, application, or platform and a provider that connects onward to carriers. Capacity is measured in channels (concurrent calls) rather than fixed lines, so a trunk can scale up or down on demand. Trunks authenticate by IP address or registration/digest credentials, and modern deployments secure signaling with TLS and media with SRTP.

Why it matters

SIP trunking decouples calling capacity from physical infrastructure, lowering cost and adding elasticity. It enables bring-your-own-carrier (BYOC), keeping existing carrier rates while moving the platform to the cloud, and lets organizations route outbound calls and deliver inbound DIDs flexibly across regions.

FAQ

What is SIP trunking? SIP trunking connects a phone system or application to the public telephone network over IP using the SIP protocol. It replaces physical lines with virtual channels that each carry one concurrent call, supporting inbound and outbound calling.

What is a SIP channel? A SIP channel is the capacity for one concurrent call on a trunk. The channel count sets how many calls can be active at once, replacing the fixed-line count of traditional telephony.