The phone screen disappears during a call making it very difficult to terminate the call! How do I get the phone screen to remain on?

Okay, if the phone screen disappears because you are just answering via the banner, then look at the top of the screen and you should see a green icon. That is the phone call. Tap on it and it will bring up the phone screen. You can see an example of it in this support document, in the section, "If you see blue, green, or red in the status bar". Status icons and symbols on your iPhone - Apple Support

Is the screen going dark when you put the phone up to your ear? That is normal behavior, however once you take the phone down from your ear, it should come back on. If it is not doing that, do you have a case or screen protector on the iPhone? One of those could be blocking the proximity sensor on the phone. it is located in the notch area, and if blocked or malfunctioning could prevent the screen from going dark and coming back on when on the phone.