can anyone give me some pointers or a list of some of these complications so that I can further research to see how its is done.

There are 3 things you should do:

1. Stop Kamailio from acting as the registrar. 2. Make sure that NAT traversal information is captured and relayed to the registrar. 3. Make sure the proxy stays in the path of future requests back to the client.

For (1) look at replacing save() with forward() in the default config. For (2) I would recommend add_contact_alias() and handle_uri_alias(), they are awesome. For (3) You can add a Path header or just have outbound proxy settings on your other servers.

The approach also depends on whether you still want Kamailio to do the authentication or if this should be handled by the registrar too.

Richard