CallKit provides the ability to block regular calls via the call extension. And CallKit also provides the ability to integrate VoIP calls with the phone's call screen/user experience etc. However the blocking aspect of CallKit seems to have no bearing on the VoIP aspect.
When the app receives a Voip Push it has to call CXProvider.reportNewIncomingCall() and that displays a call screen. If the parameters passed to reportNewIncommingCall() a handle of the .phone with a phone number registered for blocking with CallKit, the call screen still displays.
Its not use to call CXProvider.invalidate() immediately because the call screen will still display, though its only displayed for a brief time its still very very obvious and noticeable. Therefore there appears to be no way of "blocking" the call, blocking is in quotes because from the user's perspective you would not expect to see the call screen, but is there is no way of stopping it from appearing, even if that appearance is only brief?