Integrated Service Digital Network (ISDN) - Signalling System No. 7 (SS7); ISDN User part (ISUP) version 4 for the international interface - Part 21: Anonymous Call rejection (ACR) supplementary service (ITU-T Recommendation Q.731, clause 4 (1993)) (Endorsement of the English version EN 300356-21 V 4.2.1 (2001-07) as German standard)