SIP trunk

A SIP trunk is an IP service that connects a business phone system to a telephony provider for inbound and outbound calls. SIP handles signalling, while voice media commonly uses RTP or a secured equivalent, so successful registration does not guarantee two-way audio or call quality. The service may depend on the PBX, session border controller, firewall, NAT, DNS, numbering and internet access, each with separate responsibilities. Capacity is usually described through concurrent calls rather than physical lines. Design should address authentication, compatible encryption, fraud controls, emergency calling where applicable, number presentation and failure routing. Testing should cover inbound and outbound calls, transfers, caller identity, codecs, media in both directions and recovery after losing a circuit or provider. Cost and resilience depend on the contracted service and architecture.