This is a known issue
http://talk.maemo.org/showthread.php?t=40705

There is also a bug report
https://bugs.maemo.org/show_bug.cgi?id=7772

It appears bright light and a screen protector can make this happen. Use a hole punch to make a hole where the proximity sensor is in the screen protector or just flick the unlock switch to unlock the phone to answer call (power button does not work)