I have not been able to connect in France or Belgium. I'm going to Germany tomorrow and I'm fully expecting the same problem.
I have turned on data roaming and I have more than enough data available and allowed WiFi calling. But can not connect to the internet or make or receive calls. When I try, I get an English message telling me that the call can't be connected.
This is extremely frustrating and I am SO close to changing my provider.
I have a OnePlus phone. The same thing happened last year when I had a Samsung. So it's not the fault of the phone.
Thanks in anticipation for your support
Suzi