Asterisk sip dtmfmode

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DTMFmode for SIP client configuration

dtmfmode=inband

Choices are inband, rfc2833, info or auto

inband: The device that you press the key on will generate the DTMF tones. - If the codec is not ulaw or alaw then the DTMF tones will be distorted by the audio compression and will not be recognised. If the phone is set for RFC2833 and asterisk is set for inband then you may not hear anything.
rfc2833: http://www.ietf.org/rfc/rfc2833.txt
info: See SIP method info and SIP info DTMF or http://www.ietf.org/rfc/rfc2976.txt
auto: Asterisk will use rfc2833 for DTMF relay by default but will switch to inband DTMF tones if the remote side does not indicate support of rfc2833 in SDP. This feature was added on Sep 6, 2005 and is not available in Asterisk 1.0.x.

Inband DTMF won't work unless the codec is ulaw or alaw (G711). Use out of band DTMF aka rfc2833 or info.