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DTMF (Dual-Tone Multi-Frequency) is the name for the specific sounds ("dial tones") made by the keys on a telephone keypad. The choice of which method to use for DTMF over SIP is made in Asterisk's /etc/asterisk/sip.conf. Carrying DTMF over SIP can be done in one of three ways:
Inband DTMF is just as it sounds: the sound made by the key press sent in the audio channel where it is decoded by Asterisk (or whatever the termination point of the call is). Inband is only available on A-Law and Mu-Law codecs; it is generally bad practice to use Inband DTMF.
The Info method sends SIP Info messages from phone to Asterisk with the text of the buttons pressed on the phone. SIP Info tends to be a better choice than Inband because the key-presses are sent textually, independent of the audio codec. Users of Grandstream phones should use Info for the Asterisk voicemail; the other methods won't work.
The rfc2833 method uses an RTP mode defined in RFC 2833 to transmit the DTMF digits. rfc2833 is similar to Info, but will not work on Grandstream phones.
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