[Serusers] ser: cannot call behind a Nat using rtpproxy

I've installed ser 0.9.4 on Suse 10 and it's running with mysql. I can register users (I'm using serweb) but I can only make a call from a user inside the Lan (where ser is installed) to a user outside behind a Nat (Fastweb network in Italy, Natted). If i call from this user behind Nat the call does not reach the other user. Viceversa it works fine. I installed the nathelper module and rtpproxy too. I don't know if I configured it well . Have I got to open some additional port in my router on the lan where ser is running (I opened port 5060 udp and tcp only)? I've tried to use Asterix on another pc in the same lan and I don't have this kind of problems with the some users and some clients (3cx phone). Any suggestions ? thanks