Requires iOS 4.0+
CallLock is a tweak that locks your iPhone screen during a call.
It can be enabled either manually, automatically, or by proximity sensor and will prevent you from accidentally pressing buttons that could dial, mute, hold, or hangup a call by mistake. This is especially useful for people who have a faulty proximity sensor in their iPhone 4.
MANUAL MODE:
CallLock will override the default action of the Power button during a call to lock the screen instead of the normal end call action.
AUTOMATIC MODE:
CallLock will automatically lock your screen as soon as a incoming or outgoing call is connected.
PROXIMITY MODE:
CallLock will lock your screen as soon as you put the iPhone up to your ear.
CallLock is enabled in manual mode by default. To enable CallLock automatically or by proximity you must turn on the feature from inside your Settings app. No new icons will be added to your home screen.
*You do not need automatic mode enabled for proximity mode to work.
*Thanks to EvilPenguin for his assistance. Check out iBye on the Cydia Store
See More Info below for a Screenshot