Integrated Services Digital Network (ISDN) - Freephone (FPH) supplementary service; Digital Subscriber Signalling System No. one (DSS1) protocol - Part 6: Abstract Test Suite (ATS) and partial Protocol Implementation eXtra Information for Testing (PIXIT) proforma specification for the network (Endorsement of the English version EN 300210-6 V 1.2.4 (1998-12) as German Standard)