When the phone is locked (using the on/off button or if it goes to sleep) the touch screen becomes unresponsive and I cannot unlock the phone. I can unlock the phone from the lock screen when it is first turned on and the touch screen functions normally, but as soon as it is locked again this problem occurs and I have to remove the battery to turn it off as I cannot confirm switching off on the screen itself. I have tried rebooting the phone several times, increasing the delay on the long press and tried with different sim cards and no sim inserted but the problem occurs exactly the same every time.