SOLUTION******

iF your phone is blacking out, you should remove your screen protector or protective case from the phone. Removing this will allow the proximity sensor on your phone to function better. This applies to Iphone 3g, the proximity sensor is located right next the ear piece on the phone,,, it looks like small circle. Make sure that your screen protector or case is not blocking the proxmity sensor. Ever since i removed it. I have not recieved a black screen. PROBLEM SOLVED. I had to contact apple care in order to get this answer,,, i can't believe this answer is posted no where in there forums.