DTMF Tones (Dual-Tone Multi-Frequency Tones)

What is DTMF?

Dual-tone multi-frequency tones are telecommunications signals sent over voice frequencies. Those tones are used over phone lines; they are also called touch tones.

What is it used for?

Humans have required a means of consistently interacting with the system in a mechanical and repeatable manner for almost as long as there have been telegraph and telephone systems. Pulse dialing was the first widely used telephony signaling technology. Rotating rotary phones break electrical connections, and the ensuing electrical pulses are translated into commands (such as "call this number") by the receiver.

Touch tones are used for automatic telephone signaling between devices. In particular, such tones are used to control the connection between analog FXO and FXS equipment (telephones and PBX, respectively). Also, tone signals are used when the caller manually enters commands for various interactive systems, for example, interactive voice response menus.

In-band and out-of-band signaling

In-band signaling in telecommunications is the transmission of control data over the same band or channel as data, such as speech or video. In contrast, out-of-band signaling uses a different channel or even a different network to be conveyed. Participants in a phone call can frequently hear in-band signals; however, out-of-band signals are inaccessible to the user.

When dialing from a landline phone, dual-tone multi-frequency signaling (DTMF) is used to encode and transfer the phone number across the telephone line. By telling the phone switch how to route the call, the tones regulate the phone system. These control tones are transmitted in the same frequency range (300 Hz to 3.4 kHz) as the audio of the phone call over the same copper wire channel.

Because it exposes control signals, protocols, and management systems to end users and opens them up to falsification, in-band signaling is insecure. So-called phone phreaks in the 1960s and 1970s used blue boxes for deliberate falsing, in which the right tones for routing were purposefully made. This allowed the caller to take advantage of features intended for testing and administrative usage and make free long-distance calls.