The term “trunk” has been used within circuit switched telephony systems for many years. Wikipedia defines a trunk as:
A circuit between telephone switchboards or other switching equipment, as distinguished from local loop circuits which extend from telephone exchange switching equipment to individual telephones or information origination/termination equipment.
Despite its reference to a physical circuit, the term “trunk” has been used in conjunction with Session Initiation Protocol (SIP) [RFC3261] systems. A SIP trunk has been used to describe all of the following:
A service provided by service providers to enterprises, used for the purpose of interconnection to the PSTN, as a replacement for circuit based connectivity.
A SIP port on an enterprise server, for the purposes of interconnection to other server-based systems, such as voicemail servers, call centers, and application servers.
A SIP-based interconnection between IP-PBXs, for the purposes of replacing traditional TDM tie lines.