This device enables apartment dwellers to communicate with and thereby identify persons desiring ingress to an apartment building through a controlled access vestibule. The convenience and security of a conventional intercom is provided by this device, but without substations and their associated dedicated...http://www.google.ca/patents/US4172967?utm_source=gb-gplus-sharePatent US4172967 - Automatic answering device for use in live speech communication and circuit components thereof
