Integrated Services Digital Network (ISDN) - Signalling System No. 7; ISDN User Part (ISUP) version 3 for the international interface - Part 9: Closed User Group (CUG) supplementary service [ITU-T Recommendation Q.735, clause 1 (1993), modified] (Endorsement of the English version EN 300356-9 V 3-1-3 (1998-08) as German standard)