Draytek Vigor2920 Series User Manual page 350

Item
Vigor2920 Series User's Guide
Description
the keypad number you pressed and transform it to digital
form then send to the other side; the receiver will
generate the tone according to the digital form it receive.
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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