Integrated Services Digital Network (ISDN) - Signalling System No. 7 (SS7); ISDN User Part (ISUP) version 4 for the international interface - Part 18: Completion of Calls to Busy Subscriber (CCBS) supplementary service (ITU-T Recommendation Q.733, clause 3 (1997), modified) (Endorsement of the English version EN 300356-18 V 4.1.2 (2001-07) as German standard)