Integrated Services Digital Network (ISDN) - Signalling System No. 7; ISDN User Part (ISUP) version 3 for the international interface - Part 16: Call Hold (HOLD) supplementary service [ITU-T Recommendation Q.733, clause 2 (1993), modified] (Endorsement of the English version EN 300356-16 V 3.1.2 (198-08) as German standard)