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In telephone systems, multifrequency signaling in which standard set combinations of two specific voice band frequencies, one from a group of four low frequencies and the other from a group of four higher frequencies, are used. (188) Synonyms multifrequency pulsing, multifrequency signaling. Note 1: DTMF signals, unlike dial pulses, can pass through the entire connection to the destination user, and therefore lend themselves to various schemes for remote control after access, i.e. , after the connection is established. Note 2: Telephones using DTMF usually have 12 keys. Each key corresponds to a different pair of frequencies. Each pair of frequencies corresponds to one of the ten decimal digits, or to the symbol "#" or "*", the "*" being reserved for special purposes. Note 3: The standard signal frequency pairs transmitted by DTMF equipment used by the public exchange carriers are as follows:
| Button or Digit | Frequencies (Hz) |
|---|---|
| 1 | 697/1209 |
| 2 | 697/1336 |
| 3 | 697/1477 |
| 4 | 770/1209 |
| 5 | 770/1336 |
| 6 | 770/1477 |
| 7 | 852/1209 |
| 8 | 852/1336 |
| 9 | 852/1477 |
| 0 | 941/1336 |
| * | 941/1209 |
| # | 941/1477 |
| Button or Key | Frequencies (Hz) |
|---|---|
| FO (Flash Override) | 697/1633 |
| F (Flash) | 770/1633 |
| I (Immediate) | 852/1633 |
| P (Priority) | 941/1633 |