Instrument designed for simultaneous transmission and reception of the human voice.
It works by converting the sound waves of the human voice to pulses of electrical current, transmitting the current, and then retranslating the current back to sound. The U.S. patent granted to transistor (1947) led to lightweight, compact circuitry (see cell phone ). Advances in electronics have allowed the introduction of a number of "smart" features such as automatic redialing, caller identification, call waiting, and call forwarding. Telephone systems are also a primary access route for the {{link=Internet">Internet .