A PRI trunk uses the Primary Rate Interface standard to transmit as many as 23 voice channels over a single copper telephone wire. A PRI trunk provides a physical connection to the PSTN and uses a circuit-switched model to make connections. A SIP trunk uses Session Initiation Protocol to transmit data and can scale easily to accommodate as many voice and data channels as an organization requires. A SIP trunk provides a virtual connection to the PSTN and transmits voice calls as data, using a packet-switched model to make connections.