DTMF Touch-tone signaling

DTMF stands for dual tone multi frequency

                   High
                   Frequencies (Hz)

                   1209  1336  1477  1633

                697  1     2     3     A
Low
Frequencies     770  4     5     6     B
(Hz)
                852  7     8     9     C

                941  *     0     #     D

Note: The last column (1633 Hz) does not have corresponding buttons on a
      standard touch-tone telephone handset. However, many computer modems
      provide a way to send these tone pairs as part of an ATDT command.
      If the modem also provides touch-tone detection functionality then
      these tone pairs will often be decoded as A,B,C,D.