Later Alligator (Automatic Text Message)

So you get a call but you don't have time to pick up the call (or you're just not in the mood... it happens). When you get a call, there should be a field on the screen saying, for instance, 'Later'. So rather than just not picking up the call, this would allow you to automatically send a text message to the caller saying ''Call you back later'', or something along those lines.