The exchange equipment responds to the dial pulses either directly by operating relays or by storing the number in a digit register that records the dialed number. It functions by interrupting the current in the local loop between the telephone exchange and the calling party's telephone at a precise rate with a switch in the telephone that is operated by the rotary dial as it spins back to its rest position after having been rotated to each desired number. Other multi-frequency systems are also used for signaling on trunks in the telephone network.īefore the development of DTMF, telephone numbers were dialed by users with a loop-disconnect (LD) signaling, more commonly known as pulse dialing (dial pulse, DP) in the United States. Touch-Tone dialing with a telephone keypad gradually replaced the use of rotary dials and has become the industry standard in telephony. DTMF is standardized as ITU-T Recommendation Q.23. DTMF was first developed in the Bell System in the United States, and became known under the trademark Touch-Tone for use in push-button telephones supplied to telephone customers, starting in 1963. The red keys in the fourth column produce the A, B, C, and D DTMF events.ĭual-tone multi-frequency signaling ( DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. Autovon keypads were one of the few production units to include all 16 DTMF signals.
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