When the phone arrived, Miui had 11 systems on it. When you switch on for the first time, the VoWifi signal also appears next to the Telecom card. Then he immediately upgraded himself to Miui 12, there has been no VoWifi since. The switch (both VoLTE and VoWifi) is visible in the settings menu, but there is no VoWifi connection. You can also see in the call list that you are only making or receiving a VoLTE call. Has anyone else experienced this, or do I just have such "luck"?