If you don’t have VoLTE (1st code) or VoWIFI (2nd code) by default you should definitely try the code below in your dialer app.
*#*#86583#*#*
*#*#869434#*#*
After that go to Settings > Select Sim Card > Enable VoLTE/VoWIFI.
If after this you still don’t have VoLTE or VoWIFI contact your carrier as it might not support this functionality.
