[Serusers] Ser + Asterisk

Hi guys, I'm working of configurations examples, how to get Ser and Asterisk working together in peace ;) What I wan to do is to have ser doing all the hard work, registering, billing, proxying and help at nat stuff, and use asterisk to register myself at my sip providers, as I couldn't found a way to do it with ser. An idea how the things are or will be connected is... [ UAC's ] -> SER -> * / (Ser?) <-> [firewall full port redirect] -> router -> sip provider 192.168.20.x 192.168.20.1 192.168.20.10|192.168.30.10 192.168.30.1 192.168.30.x | 172.16.32.1 | 172.16.32.254 |external ip semi fixed or fixed I have all the stuff running fine if I use an IAX provider, problem is... all my providers use sip :( Does anyone has any clue to point me out ? Sip session goes fine but it just get stuck in rtp nat and I can't understand very well why. Another is the UAC's try to reach by itself the sip provider jupping over all 192.168.20.x/30.x thins goes fine if I use stun of course. My question is... Is there any way to do the same in asterisk? Like use a tun server or something, or using ser to help it to change any contact or sip headers field? HOw is the best way to acomplished that? Best Regards,