DTMF stands for dual tone multi-frequency. DTMF is a signaling system for recognizing the keys or number dialed on a telephone keypad. The first telephone systems use pulse dialing signaling. Later the system was swapped by multi-frequency (MF) dialing. DTMF is a multi-frequency tone dialing system used by the keypads in telephone and mobile phones to deliver the number or key dialed by the caller. DTMF has allowed the long-distance signaling of called numbers in voice frequency range over telephone lines. This has removed the need for telecom operator between a person calling and the person receiving a call and changed automated dialing in the telephone switching centers.


At Plivo we support two kinds of DTMF Inband  

Inband digits are passed just like normal audio tones with no special coding or markers using the same codec(G.711 audio stream) as your voice does and are generated by your phone.


Outband  

Incoming stream delivers DTMF signals out-of-audio using either SIP-INFO or RFC-2833 mechanism, independently of codecs – in this case, the DTMF signals are sent separately from the actual audio stream.