Hello alonc777,
We know it's important to have the proximity sensor working when on phone calls, and we'll be glad to help.
Let's take a look at some settings for Attention Awareness. Check to see if Attention Aware is set up on your device: About Attention Aware features on your iPhone X or iPad Pro
If you turn this off, or on, does it change the outcome while on phone calls?
Sometimes updating to the latest iOS version can help fix software issues. Please back up first, and then update.
How to back up your iPhone
Update your iPhone, iPad, or iPod touch
We hope this resolves these issues.
Take care!