This Recommendation defines the operation of the Digital Subscriber Signalling System No. 1 (DSS 1) for the support of the Terminal Portability (TP) supplementary service at the T or the coincident S and T reference point of the User-to-Network Interface of the Integrated Services Digital Network (ISDN). The Terminal Portability supplementary service allows the user to temporarily suspend an active call, and subsequently reactivate it from the same terminal equipment or from another one within the same user-network interface.