Integrated Services Digital Network (ISDN) - Signalling System No. 7; ISDN User Part (ISUP) version 3 for the international interface - Part 17: Call Waiting (CW) supplementary service [ITU-T Recommendation Q.733, clause 1 (1992), modified] (Endorsement EN 300356-17 V 3.1.3 (1998-08) as German standard)