SIP trunk

A SIP trunk connects a telephone system to a service provider using the Session Initiation Protocol. It carries signalling for establishing, changing and ending communication sessions over an IP network. Audio typically travels separately using a media transport protocol. A trunk is a logical connection, so it does not correspond directly to a single physical wire or telephone number.

Capacity, permitted destinations, number assignments and authentication are separate aspects of a trunk service. A business telephone system may need a session border controller or other network configuration to exchange calls reliably. Firewall rules, network address translation, codec choices and media paths can all affect call behaviour. SIP itself does not mean that the connection is encrypted: signalling protection and media protection must be considered explicitly. Troubleshooting should distinguish a failed session setup from a session that connects but has missing or one-way audio, because those symptoms often involve different parts of the system.