DTMF (Dual-Tone Multi-Frequency) is an audio signal combining two frequencies generated when pressing phone buttons. Also called touch-tone or tone signals, it has been used since the 1960s to transmit numeric information between phones and exchanges.
DTMF identifies signals by simultaneously sounding a low group (697-941 Hz) and high group (1209-1477 Hz) frequency. For example, "1" is 697 Hz + 1209 Hz, and "5" is 770 Hz + 1336 Hz. The reason for using two simultaneous frequencies is to prevent false detection from human voice or ambient noise. While a single frequency might coincidentally match, the probability of two specific frequencies occurring simultaneously is extremely low.
In addition to digits 0-9, * (asterisk) and # (hash) are defined as standard, totaling 12 signal types. Four extended signals A-D also exist but are not included on standard phones.
The most familiar use of DTMF is operating IVR (Interactive Voice Response) systems. When following prompts like "For XX, press 1," pressing the button sends a DTMF signal to the system, routing to the corresponding menu. DTMF is also used when entering PINs for telephone banking. In VoIP environments, signals are transmitted as digital signals (RTP events) based on RFC 4733 rather than audio, but the user experience remains the same. Learn telephone technology fundamentals in the phone number structure guide.