I’m using T-Mobile through Red Pocket (GSMT) and both VoLTE and WiFi calling seem to be working. I get the WiFi calling symbol when Wifi is connected and WiFi calling is enabled (phone app shows up in WiFi data usage list), and the standard non-WiFi call symbol with WiFi calling turned off.
However, if I set WiFi calling to “WiFi Only” for some reason it doesn’t work; I get a message telling me I need to connect to a network before I can make the call even when WiFi is connected and working.
Edit: When I checked the test menu (##4636##) is shows that WiFi calling is provisioned (although button is “greyed” out, can’t be changed), but when I check the “IMS service status” menu, I see that voice over WiFi is not available.
So I guess WiFi calling / VoWifi isn’t working after all.