ETSI ETS 300 976 Digital cellular telecommunications system (Phase 2+) (GSM); General requirements on interworking between the Public Land Mobile Network (PLMN) and the Integrated Services Digital Network (ISDN) or Public Switched Telephone Network (PSTN) (GSM 09.07 version 5.9.1 Release 1996)

standard by European Telecommunications Standards Institute, 08/01/1999