[Serusers] nat, symmetric /asymmetric UA....clarification needed

Hi Just trying to debug why some calls work and other dont, I have 2 setups, in both the UA is xlite , and is natted in both cases...however the routers differ in each case. Now I am running mediaproxy, when I debug ser, in one setups the contact header which is displayed using xlog, shows the external IP address of the router, in the other it does not, this happens after I have passed the fix_contact setting in ser.cfg. The one where it is fixed , allows me to listen to voicemail prompt from asterisk without any problems, the other...no sound. The clarification I need is this.. 1. Mediaproxy solves symmetric NAT problems with ease..Y/N ? 2. For asymmetric stuff you need to add the clients to sip/rtp-asymmetric files Y/N ? 3 If Yes the the client in this case is xlite...which is not really the asymmetric part, its the router in the middle is it not ? 4. If I place slite in the file, will it not then affect the symmetric xlite also. Now if all this fails, can I on the xlite...which seems to be behind the asymmetric setup, use STUN server, if yes, then the mediaproxy I guess will not get called, or should it.....or is it better to map the port/pathsin the router itself. As a last question, if I wanted to determine what type of NATting a device did, or I needed to debug a customers setup, whats the easiest way tks Iqbal