T1 Phone Service T1 Phone Service is a digital phone service trunk consisting of 24 voice channels or 23 voice channels and one data channel for signaling (ISDN PRI T1). Phone T1 lines are used by businesses to provide local and long distance voice service as well as other applications including video conferencing, fax, and dial-up. T1 phone lines provide increased efficiency through a PBX phone system by setting up and breaking down the phone calls faster than regular phone lines. An ISDN PRI T1 phone line offers robust features like Caller ID and Direct Inward Dialing (DID) through the D channel signaling. T1 Phone Service is also known as a Long Distance T1, ISDN PRI T1, Local T1, Voice T1, Digital T1, or T1 Trunk.