This first part of ETS 300 055 specifies the stage three of the Terminal Portability (TP) supplementary service for the pan-European Integrated Services Digital Network (ISDN) as provided by European public telecommunications operators at the T reference point or coincident S and T reference point (as defined in CCITT recommendation I.411 [1]) by means of the Digital Subscriber Signalling System No. one (DSS1). Stage three identifies the protocol procedures and switching functions needed to support a telecommunications service (see CCITT Recommendation I.130 [2]). In addition@ this standard specifies the protocol requirements at the T reference point where the service is provided to the user via a private ISDN. This standard does not specify the additional protocol requirements where the service is provided to the user via a telecommunications network that is not an ISDN. The TP supplementary service allows a user to move one terminal from one socket to another within one given basic access during the active state of a call. It also allows a user to move a call from one terminal to another terminal within one given basic access during the active phase of a call. The portability of a terminal during the idle state is part of the basic access capabilities and does not require any procedure. The portability of a terminal in the call establishment and in the call clearing phases is not possible. The TP supplementary service applies to some interactive circuit-switched telecommunication services requiring the attendance of a human being@ such as telephony@ videotelephony@ etc. The TP supplementary service does not apply to non-interactive services such as facsimile@ teletex@ mixedmode@ computer communication@ etc. However@ the network will not take any action to restrict its applicability. It is a user's responsibility to resume the call with a terminal which is compatible both with the remote terminal and the type of connection previously established. Further parts of this standard specify the method of testing required to identify conformance to this standard. This standard is applicable to equipment supporting the TP supplementary service to be attached at either side of a T reference point or coincident S and T reference point when used as an access to the public ISDN.