This standard specifies the stage three of the Closed User Group (CUG) 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 Signaling 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 CUG supplementary service enables users to form groups@ to and from which access is restricted. A specific user may be a member of one or more closed user groups. Members of a specific closed user group can communicate among themselves but not@ in general@ with users outside the group. The CUG supplementary service is applicable to all telecommunication services. Further part(s) of this standard shall specify the method of testing required to identify conformance to this standard. This standard is applicable to equipment@ supporting the CUG supplementary service@ to be attached at either side of a T reference point or coincident S and T reference points when used as an access to the public ISDN.