Ok, here is it,
BUT I can't test it, so result could be not good
The most worst situation is, that you will have to factory reset your device (myabe reflash factory image), so be prepare for this situation. If something will go wrong I take no responsibility 
Good luck and let me know if it works

Just flash it via magisk manager and restart your device. After that you could have new VoLTE switch in settings menu and check functionality throught *#*#4636#*#* menu. And ofcourse you have to have enable VoLTE on operator side, some of them doesn't do it automatically.