There are issues with the iPhone when the 3G screen goes blank or black. Press sleep/wake twice consecutively, which locks the phone, and then try unlocking it. This problem may also be due to obstruction of the proximity sensor because of the case or an errant hand. Not all iPhone users have these interference factors; even after resetting several times, working to restore build 7 is of no use. Lastly, a new firmware may help solve the issue. To try a workaround solution, this issue can be resolved by turning off the Password Lock (PIN) function in Settings. Issues like this are common in iPhone telephony.