SIP in higher education

hello,
       i have a similar configuration.I am trying to use SER as a redirect 
server. I have peer registered to SER and SER sends a redirect to that peer 
to call my asterisk box.but the problem here being it looks for the same ip 
of the registered peer with SER on the asterisk box. not able to figure out 
how can the peer and asterisk communicate independent of the SER box.

my ser.cfg
if (method=="REGISTER") {
 
                if(!www_authorize("ser domain name", "subscriber")){
                        www_challenge("ser domain name", "0");
                        break;
                }
                        sl_send_reply("200", "ok");
               break;
       };


     rewritehostport ("ip addr of asterisk box:5060"); 
          sl_send_reply ("300", "redirect");

}


please help.
Thank you,
Amy