LevelOne VOI-7011 - Manual (Page 62)

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DTMF Setting You can setup the options for DTMF
function in this page. The options include
RFC2833 (Outband DTMF), Inband
DTMF, and Send DTMF SIP info. The
default is set at Inband DTMF. If you are
making two-stage callings for extension to
PSTN, you may need to select Outband
DTMF option.
Port Setting The SIP Port and RTP Port numbers are
default at 5060 and 60000, respectively.
The RTP port number must be even
number. If you have more than one VoIP
phones under the same NAT router, it is
recommended that different RTP port
numbers be assigned to each of IP
Phones.