Being charged for repairs, phone under warranty

I took my Blackberry 9720 to the MTN store at the Phoenix Plaza for repairs. The phone switches off at any time and when receiving a call the screen says \Switch Off Device".<br> <br> The phone is still under warranty. The repair center sent me a bill to pay before they can repair my phone saying that I damaged the phone even though it is clear that I didn't. I don't mishandle my phone and there are no signs of physical damage.<br> <br> I think they are just trying to make a quick buck. Please resolve."""