[Чък Норис]

OK to make it clear, this is the clear steps to enable VoLTE:
1. Check Mobile Network Setting (Settings ==> SIM cards & Mobile Networks). If you don't have "VoLTE enabled" switch option, it means your selected region not support VoLTE, then continue with Step-2. If you have it, then proceed to Step-3.
2. Change your region (Settings ==> Additional settings ==> Region). What I've found, Hongkong and India regions support VoLTE. You can try with other regions, and update us here.
3. In the Mobile Network Setting (Settings ==> SIM cards & Mobile Networks), if the "VoLTE enabled" switch option can not be changed (in grey just like the screenshot in the OP), then you need to continue to Step-4.
4. Disable operator check by entering the code in the dialer: *#*#86583#*#* (thanks to @brigudav)
5. Now you can enable VoLTE with that "VoLTE enabled" option in the Mobile Network Setting (Settings ==> SIM cards & Mobile Networks). No need to reboot your phone, just enable and make the call with VoLTE.

And don't forget, to have VoLTE work in your device you also need to include the function in your number subscription, contact your mobile network provider.

И в крайна сметка пак к0р.