[Users] fix_nated_contact not rewriting contact?

[kjcsb]

I have the following code in openser.cfg

        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