Voice-band modems are used to transmit information via a loaded telephone line when a telephone connected to the line is on-hook. When the telephone is off-hook, the integrity of the line is checked by transmitting a pilot tone from the subscriber end to the central office end of the line, where the...http://www.google.co.uk/patents/US4521643?utm_source=gb-gplus-sharePatent US4521643 - Apparatus for transmitting information via telephone lines
