Circuit preventing bypassing of subscriber number blocking devices - is designed for PBX's with key dialling by multiple frequency system

摘要

The telephone circuit prevents by-passing of subscriber number blocking devices in PBX's having key dialling using the multiple frequency system. When a connection is made to a local public exchange and a non-authorised number is dialled, the connection is broken. a multiple frequency key dialling receiver (MFV-E1) is provided in the PBX. It receives the a.f. frequencies transmitted to the local exchange, and retransmits to an evaluation device the recognised digits. It signals in addition when the effective duration of the key operation by the subscriber (T1n) is shorter than the min. duration which the key dialling receiver (MFV-E2) in the local exchange (OVST) can directly recognise.