I have problems making WiFi Calls on my Xiaomi Redmi 9AT

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 Settings.

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.