[Serusers] fix_nated_contact not rewriting contact?

I have the following config: if (client_nat_test("3")) { #IP address in contact is private IP #Via is different to address client has contacted us from #If so then setflag7 to indicate that sender is behind NAT xlog("client nat test 3 returned true for r-uri <$ru> in route 3 so will setflag7"); setflag(7); xlog("contact before update <$ct>"); xlog("via before update <$hdr(via)>"); force_rport(); #Rewrite Contact: header to contain Sip client's public IP:port fix_nated_contact(); xlog("contact is now <$ct>"); xlog("via is now <$hdr(via)>"); }; However the value returned by $ct is the same before and after fix_nated_contact(). Is that because I'm doing something wrong, or because I've misunderstood fix_nated_contact or because the $ct variable doesn't get updated by fix_nated_contact? If the latter, how can I find out the result of fix_nated_contact? The same applies to force_rport and the via details. Any advice appreciated. Regards Cameron