Only the basic telephone services can be transmitted with the Telephony User Part because the number of protocol elements supported by all connected ISDN networks is substantially limited. Supported services include call setup, busy signaling, and call termination. Supplementary services or teleservices cannot be transmitted. Carrier services can be distinguished between the 64-kbit/s service and the 3.1-kHz audio service.Digital Subscriber System No. 1 (DSS1) signalling protocol, a uniform network interface was defined based on the standard ISDN User Part (ISUP).
The Telephony User Part is specified by the International Telecommunication Union (ITU) and standardized under Q.763.