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 only one type of DTMF:


Outband  

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