A D Channel is the signaling and control channel on an Integrated Services Digital Network (ISDN) Primary Rate Interface (PRI) T1 phone service. In a typical ISDN PRI configuration, there are 23 B channels (voice) and one PRI D channel (signaling, control, call data), this is known as a 23B+D configuration. 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. The ISDN PRI D channel 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, providing robust features like Caller ID, Direct Inward Dialing (DID), and advanced call routing. PRI D Channel is also known as the Delta Channel, Data Channel, Signaling Channel, or Control Channel.