dual-tone multifrequency (DTMF) signaling

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. 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:

Note 4: Tactical telephones have 16 keys, the extra 4 being used for precedence. For DSN (Defense Switched Network) the signal frequency pairs transmitted for the ten decimal digits and the * and , are the same as those used by the public exchange carriers. The additional four keys, corresponding to four different frequency pairs and the precedence, are as follows: