FakeHistory is a tweak released during this night in Cydia, and with its help you can falsify your contact book entries extremely easily and quickly. The tweak has a very simple menu in which you specify the type of call you want to save, the date it was initiated/received, the phone number or email address from which it came or to which it was initiated, and at the end you have the option to enter its duration. Basically you can make fake calls that look exactly like the real ones and you can fool anyone.
FakeHistory is available for $5 in Cydia's BigBoss repo.
