I remember that I tried this as well in the past, but with no configuration I could make it work. Linphone works like a charm with the SIP server in the Fritz!Box (same as the Fritz!App is trying to connect to), I recently made some phone calls from Greece over my SIP server in Germany through Wireguard and all worked as it should. So I believe now the Fritz!App has a problem when the SIP phone is in a different subnet and not connected with the VPN server on the Fritz!Box itself. So I stopped trying that because I have a working solution now. It's supporting video calls as well but I didn't try yet.