Can an iPhone app block phone calls?

Is it possible to write an application that will block incoming and outcoming phone calls? Or is the iPhone locked down too much? Thanks!

EDIT: See Rajan Maheshwari's answer below. CallKit now provides this. Even things that seemed they would never change, can change eventually.

Anything that modifies a user's ability to make or receive phone calls is going to run afoul of Apple's basic approach to third-party apps. There are a lot of things that are questionable and you might get away with. Blocking calls is clearly forbidden.

It is now possible to detect and block unwanted phone calls from iOS 10 and above.

See the CallKit framework

The CallKit framework (CallKit.framework) lets VoIP apps integrate with the iPhone UI and give users a great experience. Use this framework to let users view and answer incoming VoIP calls on the lock screen and manage contacts from VoIP calls in the Phone app’s Favorites and Recents views.

CallKit also introduces app extensions that enable call blocking and caller identification. You can create an app extension that can associate a phone number with a name or tell the system when a number should be blocked.

A number of events will potentially interrupt an application - incoming call, SMS message or calendar alert. If the user ignores the interruption your application will continue running. If not, it will terminate.

See Apple docs for more details.