As far as I know, you will only get VoLTE if your network provider gives it to you. In my case, with Orange France, the POCO F3 is NOT on their supported list, thus no VoLTE on this device.
You could manually override this like shown in the video above, and it worked for me with Android 11 (MIUI 12.5), but since Android 12 (MIUI 13) the *#*#VOLTE#*#* command does nothing, no message appears and no settings change.
I managed to contact Orange France and they activated a switch on my line, and after a couple of days VoLTE had come back (with MIUI 13)… but i only lasted a few days until it disappeared again. So I let it be…