This is a quick video on how to enable Voice over LTE and Voice over Wi-Fi for your Xiaomi, Redmi or Pocophone device. For quick reference - the dial codes you need to enter to disable carrier check are the following:
VoLTE = * # * # 86583 # * # *
VoWiFi = * # * # 869434 # * # *
This is recorded on a Poco M3 running MIUI 12.0.8 Global. Please make sure your carrier supports these features for best results. Your ROM will also need the configuration files to support your carrier for Voice over Wi-Fi - MIUI 12 does not contain all the necessary files for all carriers on all devices. If this is your situation, you may need to root your device and install a custom ROM, or wait for Xiaomi to include these for your specific device (which may never happen, knowing Xiaomi).
