Call reject SMS iPhone app

I seek to create an iPhone app that interacts with/modifies the screen that pops up during the time that an incoming call is received.? ## Deliverables When an incoming call is received, instead of just popping up with the name/phone number of the caller, a dialog box should appear. That dialog box should allow the recipient to press a button to immediately send an SMS message to the caller, such as "Ill call you back in 5." The content of the SMS message should be configurable by the user.