A system for selectively receiving incoming telephone calls to a phoneset includes an automatic answering device which answers all incoming calls by placing a simulated phone load across the phone lines. The answering device also enables a dual tone multifrequency (DTMF) receiver to receive tone frequencies...http://www.google.es/patents/US4845743?utm_source=gb-gplus-sharePatente US4845743 - System for selectively receiving telephone calls
