[Users] Force RTP proxy when calling NATed UAS

[Leonid Fainshtein]

Hi,
I would like to force RTP proxy for calls when UAC is located on the
public Internet and the UAS is located behind a NAT system. The
nat_uac_test() doesn't help in such situation because the INVITE is
issued by endpoint located on the public Internet. In other words, I'm
looking for a way to get info about the UAS. If it is located behind NAT
than I would force RTP proxy.

Thank you in advance,
Leonid Fainshtein