Integrated Services Digital Network (ISDN) - Completion of Calls to Busy Subscriber (CCBS) supplementary service; Digital Subscriber Signalling System No. one (DSS1) protocol - Part 4: Abstract Test Suite (ATS) and partial Protocol Implementation eXtra Information for Testing (PIXIT) proforma specification for the user (Endorsement of the English version EN 300359-4 V 1.3.2 (2000-06) as German standard)