[Serusers] configuration for public ser and natted pstn gateway

[Simone Cittadini]

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 ?