ETSI TS 100 976 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 7.1.1 Release 1998)

standard by European Telecommunications Standards Institute, 07/01/1999