I discovered the debug checkbox on the bottom right and looked at the log. I believe that I found the problem, which is a bug in the client software. Here is the connect request:
Request [transactionID='z9hG4bK-2857f1baa9454e83938052e5bdd3bbbc';method='INVITE'; cseq='1'; transport='UDP'; size='510'; sent '192.168.2.102:5090' -> '92.230.71.67:5060'.
As you see, the request goes out on port 5060, even though I told the client to use port 5090. Either both ends should be port 5090, or it should allow you to enter ports for both ends.
I tried a potential work-around, which would be to define the radio IP (Contact) as e.g. 92.230.71.67:5090, but the client doesn't allow the port to be entered in this field (is logical, because of the port field directly underneath).
As I stated before, I can't use port 5060, since SIP VoIP is hard wired into the routers on both ends (plus it would be common in other routers while roaming).
73,
Mitch