Dual Tone Multifrequency (DTMF)

DTMF is the tones heard when pressing a button on a telephone. Those tones are comprised of two tones played together (thus the name "dual tone"), as shown in the table below. These are defined in ITU-T Recommendation Q.23.

  1209Hz 1336Hz 1477Hz 1633Hz
697Hz 1 2 3 A
770Hz 4 5 6 B
852Hz 7 8 9 C
941Hz * 0 # D