Telephone exchange graded service system - sends caller audible signal different from usual tones if call type is unauthorised

摘要

The telephone exchange restrics the type of call, e.g. local, trunk, intercontinental, that a subscriber make by comparing his dialled regional prefix with stored authorisation codes. Connections are rejected if the comparison is negative from the caller's viewpoint. The caller is informed that his call has been rejected by an audible signal and/or an announcement differing from the normal audible signals and messages. If the subscribers are connected to a private branch exchange the comparison that decides authorisation or rejection takes place at the exchange to which the branch exchange is connected.