I believe the VoLTE toggle's presence is controlled by whether there is any fallback from VoLTE. Apparently a substantial number of carriers don't offer anythin

I believe the VoLTE toggle's presence is controlled by whether there is any fallback from VoLTE. Apparently a substantial number of carriers don't offer anything other than VoLTE at this point. Turning off VoLTE when you are with an affected carrier is equivalent, at that point, to just turning off calls altogether.