[Serusers] configuration for public ser and natted pstn gateway

I've the following configuration : internet --- ser -||-- asterisk where ser has two network interfaces, one with public ip and one with 192.168.1.1, asterisk sits in the 192.168.1.0 net, authorization and forwarding of calls from internet works well with this ser.cfg (I omit parameters and obvious stuff): if (uri==myself) { if (method=="REGISTER") { if (!radius_www_authorize("")) { www_challenge("", "0"); break; }; save("location"); break; }; lookup("aliases"); }; if (method=="INVITE") { record_route(); }; rewritehost("192.168.1.101"); <--- asterisk ip if (!t_relay()) { sl_reply_error(); }; but of course you can't hear nothing with this config... Googling I only find configurations relative to the ser machine also being natted, can someone tell me what I need to do in my particular case ?