Using your client over VPN does not work with the server name. In our case, I can not set "pbx.nettecture.local" on the client as server. If i resolve the ip address manually (i.e. in a command line, ping pbx.nettecture.local), it resolved. If I then enter the ip address - it connects. Something is wrong with the address resolution, obviously. The same issue happens with x-lite, so I am not sure it is your client or your server.