I'm looking to design a simple app that categorizes contacts based on priority, and allow a different incoming screen to be displayed when that contact calls in depending on the contact's priority. Is this achievable by using the Core Telephony library, or is this simply something that's forbidden?
Apps can only modify things within their own sandbox. The incoming call screen, is part of the phone app and therefore can't be modified. The only way to do this would be on a jailbroken iPhone, and it would be quite difficult.
There is no way to get an app like that on the Apple App Store.