Session Initiation Protocol
SIP is used for establishing a voice communication session on a data network (for example over the Internet). A SIP “session” might be a regular VoIP phone call between two participants or a multi-party conference call. A SIP Trunk provides the same service you get from a traditional analog phone line. The difference is, instead of being a physical wire, a SIP Trunk is a “virtual” phone line which is provided by a SIP trunk provider. They use your data circuit (T1, Cable Modem, DSL, Ethernet over Copper, etc) to connect your phone system back to their network.