For Xiaomi:

To unlock VoLTE: open your phone app and dial *#*#86583#*#*
To unlock VoWifi: open your phone app and dial *#*#869434#*#*

Now go into Settings > Sim Cards & Mobile Networks. You will see an option called "VoLTE enabled". Tap the switch to enable it. Scroll down further and you will see another option called "Make calls using Wi-Fi". Tap the switch to enable it. You can then choose whether the phone should prefer network or wifi right below.