You have to use nathelper+rtpproxy as soon as one of the clients is behind NAT. E.g. if the public client calls the natted client, you also have to use rtpproxy. Thus, you have to setup a reply route.
nat_uac_test detects only if the caller is natted. To detect if the callee is NATed, use the nat_flag parameter: http://lists.iptel.org/pipermail/serusers/2003-December/004412.html
Anyone knows how to find out if a callee in a mid-dialog call is behind NAT?
Thanks, Richard