I have just bought a second hand 16gb iphone 3g. I have inserted a 3g pay and go sim card but all i am getting is the screen saying 'No sim card inserted. Insert a valid sim with no PIN lock to activate iphone.' It says no sim in top left corner, keypad locked in centre at top, i tunes symbol below, and slide for emergency at the bottom. I can not do anything with it. I have tried resetting it by holding home and sleep buttons but it starts back up with the same screen as detailed. I have plugged it into USB and accessed DCIM which doesn't show anything at all. Any ideas on how i can get this up and running would be greatly appreciated.