Telephone line monitoring circuits therefor, e.g. ring detectors

Substation equipment, e.g. for use by subscribers > Automatic arrangements for answering calls; Automatic arrangements for recording messages for absent subscribers; Arrangements for recording conversations > Recording arrangements > Telephone line monitoring circuits therefor, e.g. ring detectors

Substation equipment, e.g. for use by subscribers (subscriber services or facilities provided at exchanges H04M3/00; prepayment telephone coin boxes H04M17/00; current supply arrangements H04M19/08) > Automatic arrangements for answering calls; Automatic arrangements for recording messages for absent subscribers; Arrangements for recording conversations (centralised dictation systems H04M11/10) > Recording arrangements > Telephone line monitoring circuits therefor, e.g. ring detectors