DTMF
full spelling ： Dual Tone Multi-Frequency
DTMF is an acronym for Dual Tone Multi-Frequency which is also known by the trademarked term Touch-Tone. This system is used by push-button telephones for dialing and in network signaling. In DTMF, pressing a key on the phone's keypad generates two simultaneous tones, one for the row and one for the column. These are decoded by the exchange to determine which key was pressed.
DTMF was developed at Bell Labs. Today DTMF is used by all modern telephones. The Touch Tone system also introduced a standardized keypad layout familiar to us today, with 1 in the upper left and 0 at the bottom.