SIS SS 63 63 33-1991 Telecommunications equipment — Private Branch Exchanges (PBXs) — Signalling requirements in digital interface for exchan- ge line with direct dialling-in service
1991SIS SS 63 63 33-1991 Telecommunications equipment — Private Branch Exchanges (PBXs) — Signalling requirements in digital interface for exchan- ge line with direct dialling-in service