[Serusers] grandstream configuration -- help

Hi users , I am using ser-0.96 , and every thing seems to work fine . but when I make calls from grandstream ATA .. its is working properly but the problem is when the other party hung the phone SER cannot send BYE packets to grandstream NAT ip . It is sending to grandstream private-ip which inturns delayed for acct.stop packet unless grandstream hung the phone x-lite, is O.K and its sending to SER as tel.no@Nat-ip-address but grandstream is sending to SER as tel.no@private-ip-address so in record-route SER is strictly following this and making some small mistake,, like sending Bye request to private-ip How to solve this : -) Thank You. Regards, Ravi.