DTMF is standardized in CCITT recommandation Q.23 and in CEPT recommandation T/CS 46-02.
DTMF uses two groups of four frequencies each.
Each combination of two frequencies defines a symbol. DTMF is used on many telephones.
| 1209 Hz | 1336 Hz | 1477 Hz | 1633 Hz | |
|---|---|---|---|---|
| 697 Hz | 1 | 2 | 3 | A |
| 770 Hz | 4 | 5 | 6 | B |
| 852 Hz | 7 | 8 | 9 | C |
| 941 Hz | * | 0 | # | D |
On American telephones symbols 2 to 9 are accompanied by other symbols. This enables advertising phone numbers like 1-800-FICTION.
1 | ABC 2 | DEF 3 |
| GHI 4 | JKL 5 | MNO 6 |
| PQRS 7 | TUV 8 | WXYZ 9 |
* | 0 | # |