The DTMF mode is a setting that governs how Dual Tone Multi Frequency signalling is to be performed.

Mode Description
Compatibility Use RFC2833 by default but switch to inband when reciever does not support RFC2833.
RFC2833 Send DTMF information as RTP messages.
SIP INFO Send DTMF information as SIP messages.
Inband DTMF tones are generated by the calling device.