The multi-frequency dialing (DTMF), Multi Frequency Code (MFC), is a signaling method of telephone connections. In this method, discrete tone frequency pairs are assigned to the dial digits on the dial keypad.
Each dial pad key is coded with one frequency for the column and one for the row. When a call is set up, the combination of the two frequencies is transmitted from the subscriber telephone to the local exchange. The audio frequencies are still available after the connection is established and can be used for additional information transfer. On the receiving side, the information transfer can be selected and used accordingly, for example for direct marketing, consultations, acquisitions or orders.
The multi-frequency dialing method is also referred to as frequency dialing, tone dialing or dual-tone dialing. It is defined in ITU Recommendations Q.23 and Q.25.