VoLTE is to help with the sound quality on a call using your mobile network
WiFi Calling is when you do not have a mobile signal but are connected to WiFi in which case the call is routed over your WiFi and not on a mobile signal ( assuming your carrier supports it )
No need to turn off WIFI calling - phone will only use it ( if you have it set up ) if you do NOT have a mobile signal but WiFi connection is available If you have a mobile signal then phone will use that
The VoLte is something to do with the phone always trying to use a 4g ( Lte ) connection on the mobile signal for better sound quality