Draytek Vigor2130 series User Manual page 235

Also, you can specify each field for your necessity. It is
recommended for you to use the default settings for VoIP
communication.
Symmetric RTP – Check this box to invoke the function.
RTP
To make the data transmission going through on both ends of
local router and remote router not misleading due to IP lost
(for example, sending data from the public IP of remote
router to the private IP of local router), you can check this
box to solve this problem.
Dynamic RTP Port Start - Specifies the start port for RTP
stream. The default value is 10050.
Dynamic RTP Port End - Specifies the end port for RTP
stream. The default value is 15000.
RTP TOS – It decides the level of VoIP package. Use the
drop down list to choose any one of them.
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Vigor2130 Series User's Guide