Integrated Services Digital Network (ISDN) - Completion of Calls to Busy Subscriber (CCBS) supplementary service - Functional capabilities and information flows; English version ETS 300358:1995
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
