Hello there Dr. Dave,
It sounds like your proximity sensor is not turning the screen black to prevent buttons from being pressed. I recommend the troubleshooting from the article named:
iPhone: Hardware troubleshooting
- Verify mute or speaker have not been activated, just prior to placing your ear over the receiver.
- While on a call hold your hand over the top third of the display, and verify that the screen is locked or goes to sleep.
- Try turning iPhone off and then on again.
If the issue persists, I would next close all the running apps on the phone:
iOS: Force an app to close
Double-click the Home button.
Swipe left or right until you have located the app you wish to close.
Swipe the app up to close it.
When you have done that and restart the phone and test the issue again:
iOS: Turning off and on (restarting) and resetting
Thank you for using Apple Support Communities.
All the very best,
sterling: Thanks for your interest. I did all you recommended by the darn buttons are way, way to sensitive. It is very difficult, for instance, not to trigger the speaker phone. Just putting it next to my cheek, let alone touching it, puts it on speaker. [Hey Apple: why don't you let me control sensitivity in Settings. Better yet, provide a button to let me lock the screen while phoning. Or better yet, provide a lock/unlock button and let me pick the default. But what do I know?]
Thank you for the update Dr. Dave and thank you for using the Apple Support Community. Due to the nature of your issue you may find more information by using another one of Apple's support resources - https://getsupport.apple.com/GetproductgroupList.action.
sterling: Your very first sentence was correct -- the proximity sensor is belly up and the instrument has to be replaced. The bummer is that I did not know the sensor was there and it has been broken since day one. I could have traded it in when I bought it a couple of years ago. Not now, though. Oh, well. Thanks for your help.