[Serusers] Registrations behind NAT problem.

Hi, Im facing some weird problem, when users register from behind NAT. Let me try to break it down. First of all, Im using ser 0.9.6 and my configuration file is almost identical to that of mediaproxy+nat config file from getting started bundle. I have asked my friends and colleagues to test my SIP-based application behind NAT with their ADSL modems and strange thing happens: 1. While one guy (say user1) has ADSL modem, which gives him address of 192.168.0.3, he can register w/o problem. Contact header <sip:user1@192.168.0.3:5060;transport=udp>;q=1.0 is sent in REGISTER request, and received back with 200 OK as <sip:user1@192.168.0.3:5060;transport=udp>;q=1.0;expires=3600;received="sip:61.47.x.x:5060" (i have put "x" by myself) 2. Another guy (user2) has ADSL modem connected to a wireless access point. The application sends in Contact: <sip:surat@10.0.0.12:5060;transport=udp>;q=1.0 but gets back in 200 OK (attention !!!): <sip:surat@124.120.x.x:41854;transport=udp>;q=1;expires=3600;received="sip:124.120.x.x:43041" It gets Contact with a public IP back, which is not the case with user1. How come? I *wonder* why this is happening... Some kind of weird NAT? I'd really appreciate if anyone can shed some light on this issue. Bests, Andrey.