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If you can't use WiFi Calling, there may be several causes of the problem.
Press Settings.
Press SIM cards & mobile networks.
Press the required SIM.
Press the indicator next to 'Make calls using Wi-Fi' to turn the function on or off.
Press the Home key to return to the home screen.
Press About phone.
Press MIUI version. If a new software version is available, it's displayed. Follow the instructions on the screen to update the phone software.
Press and hold On/Off until your phone restarts.