How do I turn VoLTE on / off on my mobile phone?

Activate your phone options for 4G network and VoLTE calls. This is usually done in Settings > About the phone> Connections> Mobile networks. The order and menu names may differ depending on the model of the smartphone. In the mobile network menu, there is usually a radio button to activate and deactivate VoLTE calls. When activated, the VoLTE icon will be displayed on the home screen.
If you want to disable VoLTE on your mobile phone, enter Settings and then turn off the function via the corresponding radio button from Connections / Mobile networks.