Apple iPhone 3G

I am at a loss. I have had a Iphone 3g for a while now. I have traded it in for a replacement now three times. What seems to happen is whenever I get a call (although not EVERY time, just a decent amount) after the call is over I go to hang up and the screen is black and will not come back. Despite my best efforts it takes powering the unit down and back on to get it to end calls. With this being the third unit I can't seem to imagine that it is three faulty phones with exactly the same problem? Another symptom is that the phone will take pictures of nothing, AND pictures of its own screen (like a screen shot)

It is in a carrying case although it is a case specifically designed for the Iphone, but it being in a case may account for the p...
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