DTMF (Dual-Tone Multi-Frequency signaling)
Dual-Tone Multi-Frequency (DTMF) signaling is a system by which audible tones are used to represent buttons being pressed on a keypad. Frequently referred to in the United States as "touch tone" (after the Touch-Tone trademark used when the transition from pulse dialing to DTMF began), DTMF makes it possible to signal the digits 0-9 as well as the letters "A" through "D" and the symbols "#" and "*". Few telephone keypads include the letters, which are typically used for control signaling by the telephone network.
Computers may make use of DTMF when dialing a modem, or when sending commands to a menu system for teleconferencing or other purposes.