Help! Why am I getting a black screen when I try to use my phone app? Can't make calls or send texts.

Background: I recently updated to the latest iOS 13x (can't remember if it's 4 or 5). A couple of days ago, after airdropping some photos in a message, a left-pointing navigational arrow on the upper right side (only) of the screen. When I swiped left, a thumbnail-size black box appeared, but I got rid of it by swiping right (leaving the arrow still showing) and went about my business. The arrow continued to pop up in the phone app until I dismissed the app.

Last night I made a Facetime call with no problems except the arrow/box. Then, when I tried to make a phone call today, instead of the usual phone Keypad screen with the red hang up button I got a blank black screen. The person receiving the call answered; I don't know if she could hear me. I hung up and was able to dismiss the phone app but not the blank black screen. Same thing with two more test calls and one attempted Message text.

Summary: If I open the phone app, I can access both Contacts and the Keypad. A contact search produces a normal contact page, but if I try to make a call or send a text, all I get is a black screen. When I dismiss the phone app, the black screen reappears, blocking half the screen, and the only way I can get rid of it is to press Home. It appears when I dismiss any other app as well.

QUESTIONS: (1) How can I fix this glitch, which never happened until I updated? (2) If I need to uninstall the phone app, how do I do that? (I'm very low tech and never know where to find anything.) Thanks so much for any help! Apart from not being able to make phone calls, I feel like I'm in 2001, and not in a good way.