Any ideas? I get the feeling I'm missing something obvious!
Perhaps this could help:
https://www.kamailio.org/docs/modules/5.5.x/modules/nathelper.html#nathelper...
This involves adding additional data to the Contact URI which I want to avoid if possible.
Also, unless I misunderstand, it seems that the alias parameter is used to populate the $du which I can do effectively, it's the source that I'm having issues with or having it use an existing socket to that same destination.
Ross