I have activated my OnePlus 7T on Google Fi and have been using it for about 2 or 3 months now. I'm not sure about the WiFi calling, I don't see an option for it in the Google Fi app, and I don't see an option for it in System Settings anywhere. I can tell you that you can't install Google's Phone app. I don't know why, it shows up in my Play Store Library, but I don't have the option to install it. Service is still good. Sometimes calls do take time to actually start dialing and there are a few locations in my area between cities where my service will drop but the radius that I lose service in is rather small. I didn't have the service dropping issues with my Pixel on Google Fi.