The Primary Rate Interface (PRI) is a telecommunications interface standard used on an Integrated Services Digital Network (ISDN) for carrying multiple DS0 voice and data transmissions between the network and a user.

PRI is the standard for providing telecommunication services to offices. It is based on the T-carrier (T1) line in the US and Canada, and the E-carrier (E1) line in Europe. The T1 line consists of 24 channels, while an E1 has 32.

PRI provides a varying number of channels depending on the standards in the country of implementation. In North America and Japan it consists of 23xB (B channels (bearer channels)) and 1xD (D channel (data[1] channel)) (23 64-kbit/s digital channels + 1 64-kbit/s signaling/control channel) on a T1 (1.544 Mbit/s). In Europe and Australia it is 30B + D[2] on an E1 2.048 Mbit/s. One timeslot(0) on the E1 is used for synchronization purposes and is not considered to be a B or D channel. The D-channel typically uses timeslot 16 on an E1, vs timeslot 24 for a T1.

Although still the primary method for most corporations and businesses today many organizations are migrating to SIP trunking instead.  The Advantages of the past such as Direct in Dialing have given way now that Sip is available.  Pri's can simulate Anaog lines via a PBX (phone system) but more and more people are migrating to SIp becuase of the high cost of the Local loop.  The local loop is the pair of wires from the Local Exchange Carrier which send signals from the customer presence to the Central office (ie Verizon) and on to the providing carrier.  (Like AT&T, Windstream, Etc.)  In addition Pri's provide 23 lines whether you need al of them or not whereas SIP is done on a channel by Channel basis so you only pay for what you need.