Glossary definition of DTMF

Also known as: Dual Tone Multi-Frequency

Dual Tone Multi-Frequency (DTMF), also known as 'Touchtone', is the type of signalling now used by all modern telephony equipment.

DTMF is the name given to the system of audible tones generated by a user when dialling numbers on a telephone, and DTMF phones usually have 12 keys (i.e. the ten decimal digits plus the symbols '#' and '*'). Each phone key generates a different pair of audible frequencies-one high and one low frequency, so that they cannot be imitated by a voice-which explains the term 'Dual Tone'. At the exchange or receiving equipment these tones are then decoded, to determine which keys had been pressed.

In addition to their use for dialling, these tones are also used to input data after a connection has been made, for example when activating voice mail, selecting a service from automatic answering systems, or banking by phone.