My girlfriend has been having an issue with missing contacts on here iPhone 4S for a while. She be looking for a number and the contact that was on there before is now gone. This is not happening to every contact, just some of them. This issue happened on iOS 5 and continues to happen on iOS 6. She uses Microsolft exchange server for work and has contacts from there, as well as when she adds contacts on the phone itself. Is there any way to stop her phone from deleting random contacts?