SIP Trunk

A virtual connection between a PBX and a service provider that replaces traditional ISDN or analog phone lines, carrying voice calls as SIP signaling and RTP media over IP. A SIP trunk carries multiple concurrent calls unlike a single physical line. Providers charge per concurrent channel or per-minute usage. SIP trunks require careful firewall and NAT configuration since both signaling (port 5060) and media (RTP, high port range) must pass.