iphone 5 6.0.2 suddenly can't end calls

Just today this started:

Initiated a call or received a call, removed from face, see screen, tap "end call" button 1 time, nothing happens. Button doesn't even register a tap.

Tap button again, button registers tap, goes to black screen. After a beat or so, screen comes back. Call did NOT end. End call button does not end the call, ever.

Sometimes, tapping the 'end call' button will make the screen go black and it won't come back. Call is still going. Have to tap the home button to get the screen to come back.

Reproduced in low light (thought it was the sensor) and full sunlight. (same behavior)

Tapping the speaker icon, setting the phone to speaker phone THEN hitting 'end call' makes it work as usual. So it's not that the touch screen doesn't work.

I've reset the phone many times.

I've turned off alarms and push.

Suggestions?

Thank you!