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
http://support.apple.com/kb/TS2802
- 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
http://support.apple.com/kb/ht5137
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
http://support.apple.com/kb/ht1430
Thank you for using Apple Support Communities.
All the very best,
Sterling