ISUP stands for ISDN User Part. It is a protocol that defines the messages and procedures for setting up, managing, and releasing voice calls over the SS7 network. ISUP is used for both fixed and mobile telephony services, such as PSTN, ISDN, GSM, and UMTS. ISUP is one of the layers of the SS7 protocol stack, along with MTP, SCCP, and TCAP.