My friend has a iPhone4 and today very strange problem appeared in his cell. He received a call and when he tried to receive the call, suddently iPhone black screen appeared and started to vibrate continuously. He had no idea what caused the problem. Then for few minutes tried to google and search the solution and then suddently I hit an idea and tried a solution which worked fine after that iPhone hasn’t shown any such problem.
1. Hold the power button (at top) and home button simultaneously for few seconds (around 10 seconds).
2. After few seconds, After iphone black screen, you will see apple logo in the screen and then iPhone will be function as before, as if normal.
Some people in forum reported that continuous vibration will drain your battery and then phone will be powered off automatically. After that you can recharge it normally and start again as normal.
Hope it helps someone.