My phone screen goes black when I am calling and I cannot use any call functions. What can I do?

[AC Question]

My phone screen goes black when I am calling and I cannot use any call functions.

I have a Motorola Droid Ultra with minor cracks in one corner of the screen. Whenever I place or pick up a call, the screen goes black and I cannot hang up, enable speakerphone, use the keypad, etc. I have no case on the phone in case that helps.

Any help is appreciated.