Duplicate port with Draytek Vigor3300V router

I’m having some problems with IP phones registering to my Asterisk server through a Draytek Vigor3300V router at a remote site.

IP Phones <–> Draytek Vigor3300V <-- Internet --> Local Router <–> Asterisk

I found that some IP phones have the same UDP port registered to the Asterisk server (from “sip show peers” command). For example, both extension 856 and 857 can be registered from the same UDP port 26085, extension 839 and 854 are registered from the same UDP port 14085, etc. These collided extensions tend to change over time, so it’s not always the same two extensions using the same port. Other phones connected through other routers does not have the same problem.

Does anybody know what the problem might be? I really appreciate your help.