SIP stands for ‘Session Initiation Protocol’ which is the protocol that controls setting up the call and tearing it back down again. A SIP trunk is a virtual telephony line that connects your office to the outside world using a data connection instead of an ISDN or analogue phone line. SIP trunks carry your voice calls like an ISDN circuit would, generally at a lower cost. SIP calls between two SIP-enabled sites are zero cost as the call remains on-net.