If you get an error code of “B050” when trying to activate your “Google Fi” service, it is because you are identified as not being in the US right now. This was confirmed to my by Google Support on the phone a few minutes ago.
Whether this is being identified by your IP address or your phone’s location, I can’t tell (yet). Could be that I bring up a VPN to check… 😉
2020-02-19 Update: I was just too curious, so only a day later I quickly wrapped up a VPN with a source IP from the US (thanks, AWS! :-)). And, presto, I could activate Google Fi. So it is crystal clear that they identify you by your IP address.
I could make calls via VoWiFi, but when I disabled WiFi I couldn’t attach to the mobile network. This is probably because I’ve never joined my “home (mobile) network” yet. I’ll be in Florida soon, then I will join the mobile network there. And I’m sure when I return to Germany I’ll be able to join German mobile networks as well…