Draytek Vigor2130 Series User Manual page 191

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. To make the
RTP
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