dual tone multi frequency

from The Free On-line Dictionary of Computing (8 July 2008)

Dual Tone Multi Frequency
DTMF

   <communications> (DTMF, or "touch-tone") A method used by the
   telephone system to communicate the keys pressed when
   dialling.  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.

   (1995-03-28)