A transmitter-receiver comprises a transmitter, a transmitter circuit, a receiver circuit, a receiver, a jack, and a switch. The transmitter circuit converts a first audio signal to a transmit signal and transmits the transmit signal. The receiver circuit extracts a second audio signal from a received...http://www.google.co.uk/patents/US5475872?utm_source=gb-gplus-sharePatent US5475872 - Portable telephone with external transmitter-receiver connections
