That is rough! It would be hard to explain to your friends that you didn't actually hang up on them on purpose if your cheek is hitting the end call button. It sounds like there might be an issue with your proximity sensor. Can you verify that the sensor is enabled by following these steps?
From a home screen, tap Phone
Tap Menu icon.
Tap Call settings.
Tap Turn off screen during calls to enable or disable.
Just checking in here to see if you are still having issues. Let me know
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