An improved combination of a hardware and digital signal processing filter for detecting pick-up of a telephone call, solely through audio information on the telephone line, is disclosed. The apparatus employs a high gain band pass filter (28) with no automatic gain control, the output of which goes...http://www.google.com.au/patents/US4477698?utm_source=gb-gplus-sharePatent US4477698 - Apparatus for detecting pick-up at a remote telephone set
