The acronym “SIP” stands for Session Initiation Protocol. SIP Trunk is a method of delivering telephone and other unified communications services over the internet. It works with a SIP enabled on premise or hosted PBX (private branch exchange). SIP Trunk replaces traditional telephone lines or PRIs (Primary Rate Interface). SIP defines the messages that are sent between endpoints and it governs establishment, termination and other essential elements of a call.

In short, SIP Trunk is a technology that allows businesses to make voice and multimedia calls over the internet instead of traditional phone lines.