When using my iPhone XR with a Lifeproof Fre case the screen goes black when I am using speakerphone, the phone is away from my ear, or when I have to press certain numbers to get somewhere on the call menu.
iPhone XR, iOS 12
Perhaps the proximity sensor is not working properly. (just to make a test, try to pull out the case).
Sometimes a forced restart can help in such a situation too:
Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then press and hold the Side button until you see the Apple logo. When it appears, relase the Side button, wait for your iPhone to restart, then test it.
If you get no improvements, since your XR is still covered by warranty, have it inspected at an Apple Store or Authorized Service Provider: Apple–LocationSelector
Regards
Giulio
Screen goes black in calls: iPhone XR, Lifeproof Fre case
