Integrated Services Digital Network (ISDN) - Signalling System No. 7; ISDN User Part (ISUP) version 4 for the international interface - Part 2: ISDN supplementary services (ITU-T Recommendation Q.730 (1999), modified) (Endorsement of the English version EN 300356-2 V 4.1.2 (2001-07) as German standard)