[Serusers] SER Proxy challenge

[Sing]

I found out that the proxy_challenge work fines if the UA has a real ip or if
the there is only one UA register to the server within the network. The
problem I found out is that the proxy_challenge will only send to port 5060 or
the sip port the UA defined, but if the UA is behind NAT and the NAT port is
not the 5060, the proxy authentication won't pass to the UA, so the UA can't
dial out. Is there any method to solve this problem? 

Sing