If you are using a Xiaomi phone and can send/receive text messages and use data, but cannot call, then please;
Dial * # * # 86583 # * # * ( No spaces required ).
You should see a “VoLTE carrier check was disabled” or “VoLTE carrier check was enabled” message after dialing the *#*#86583#*#* code.
You need to see the “VoLTE carrier check was disabled” message in order to switch VoLTE on.
If you see a message stating that the carrier check was enabled, please dial this code again until you see the “VoLTE carrier check was disabled” message.
Once you have seen this message, please test the calling feature again (and then reboot your device / retest the calling feature again if there is no change).