Draytek Vigor2920 Series User Manual page 297

Item Description
This function is very useful when the network traffic
congestion occurs and it still can remain the accuracy of
DTMF tone.
SIP INFO- Choose this one then the Vigor will capture
the DTMF tone and transfer it into SIP form. Then it will
be sent to the remote end with SIP message.
Payload Type (rfc2833) - Choose a number from 96 to 127,
the default value was 101. This setting is available for the
OutBand (RFC2833) mode.
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Vigor2920 Series User's Guide