How to enable?:
  Run the below code on your phone dialer manually:
*#*#86583#*#*

 

After that, the message VoLTE Carrier check was disabled will appear, activating VoLTE.

Next go to Settings / SIM cards and mobile networks / Use VoLTE
If the device is a Dual SIM the VoLTE option will be found in Settings SIM cards and mobile networks SIM1