A PRI Circuit is a Primary Rate Interface (PRI) phone service consisting of 23 voice or data channels (B channels) and one ISDN PRI line channel for signaling (D channel). ISDN PRI service is used by businesses to provide local and long distance voice service as well as other applications including video conferencing, fax, and dial-up. ISDN PRI T1 provides increased efficiency through a PBX phone system by setting up and breaking down the PRI phone calls faster than a regular voice T1 or phone lines. An ISDN T1 offers robust features like Caller ID and Direct Inward Dialing (DID) through the D channel signaling on the ISDN PRI line. PRI T1 lines are also known as T1 Phone Service, Digital T1, Voice T1, Local T1, Long Distance T1, or T1 Trunk.