Dual Tone Multi Frequency (DTMF) – also frequency dialling, is used for signalling between telephony devices and switching centres in the voice signalling band. Every digit is represented by a dual tone sent in the voice band. In times of VoIP and using compressed codecs (which can cause problems in the transfer of dual tones) in 2000 an RFC was introduced (now RFC4733, original RFC2833) by the IETF regarding DTMF RTP Payload. The suggested solution is that in VoIP dual tones get transferred in the RTP band, not in the voice band anymore. So, the transfer of dual tones is not influenced by the used voice codec any longer. DTMF is also referred to as dial tones and are mandatory for using voice menus / IVRs.
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