Integrated Services Digital Network (ISDN) - Signalling System No. 7; ISDN User Part (ISUP) version 3 for the international interface - Part 19: Three-Party (3PTY) supplementary service [ITU-T Recommendation Q.734, clause 2 (1993), modified] (Endorsement of the English version EN 300356-19 V 3.1.3 (1998-08) as German standard)