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 |