iphone XS screen goes black during call

(1) Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

(2)  If that does not help, then there may be an issue with the proximity sensors on the iPhone that detect your ear/face so that it can shut off/turn on and disable/enable the screen. I recommend that you contact 👉 Official Apple Support OR you can always get your iPhone examined at the Apple Store 👉 Genius Bar Reservation and Apple Support Options or at an Authorized Service Center for further diagnosis and evaluation.

Axel F.

You can use the same instructions in my previous response. Note that point 2 asks you to contact Apple Support as well.

Axel F.