Why do we use DTMF


What is DTMF?

DTMF (Dual Tone Multifrequency) means the transmission of key tones via the telephone network to the remote station.

In everyday life you use it very often and yet it is a functionality of all telephones that is probably one of the least paid attention to: The key tones - alias: DTMF.

DTMF for communication with speech dialogue systems (IVR)

DTMF tones are key tones that are sent by your phone to enable certain functions. DTMF is mostly used for the remote query of mailboxes, the control of dialogue systems (such as an IVR) in call centers or the dialing into a conference number via PIN entry. You are probably familiar with the announcement "To speak to customer support, press 1" - when you press the button, you then signal that you would like to be connected to customer support. The system recognizes the key pressed by the tone that is transmitted.

DTMF is an English term. It is called "Dual Tone Multifrequency" and is standardized worldwide. In German there is mostly talk of multiple frequency dialing (MFV).

How does DTMF work?

And this is how DTMF works: When you press each digit, tones are generated that consist of two different frequencies. The answering system recognizes these tones and executes the corresponding function - for example playing your mailbox messages or establishing a connection to the desired contact person. The tone frequencies result from the key position - and this is how the familiar arrangement of the telephone keys came about.

The history of DTMF

DTMF was introduced as "Touch Tone" by Bell Laboratories in the USA in the 1960s. Before DTMF found its way into telecommunications, pulse dialing (IWV) was common practice. In this process, the electromechanical impulses were generated by the dial, which could be interpreted by the so-called "voters" in the switching centers. With DTMF, on the other hand, electronic circuits are required.

Although the switching centers in Germany were digitized in 1990, they still recognize both methods today. Telephones that support the old pulse dialing method are still being built for the international market today. The reason: In some countries modernization has not yet taken place - the switching systems there only understand the old system.


And even today we are facing a change again - the shutdown of the old networks in favor of Voice Over IP. In order to make DTMF possible with Internet telephony and thus the use of IVRs, remote inquiries, etc., VoIP processes are used that transmit multi-frequency tones encoded in data. The sound is no longer decisive for the use of these functions, but the data sent.