An automatic telephone dialer is described which includes a dial tone detection circuit. The dial tone detection circuit responds to a series of tones which are prerecorded on a magnetic tape. The dial tone detection circuit operates to detect the tones and to provide a signal suitable to activate...http://www.google.co.uk/patents/US4145581?utm_source=gb-gplus-sharePatent US4145581 - Automatic telephone dialer for use in intrusion detection systems
