Integrated Services Digital Network (ISDN) - Completion of Calls to Busy Subscriber (CCBS) supplementary service - Digital Subscriber Signalling System No. one (DSS1) protocol - Part 1: Protocol specification
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]
