Nowadays I am thinking about a mobile application (specially for iPhone) and trying to build it for personal use and of course will allow to others too. I want to develop such application, suppose I have iPhone device.
My iPhone get missed call in the absence of me and in that that case suppose I missed some urgent call and I really did not want to miss them. So that my device should have automatically sent messages to them all that I could not receive those calls in due to my absence and message would be like "I am unavailable right now, please call me back or wait for my call".
So all that was the idea of mine I want to develop such mobile application, I would keep each think in my mind like don't send unnecessary reply even not needed. I'll look to the UI part also. That's why I am studying AddressBook frame and all related tutorial, and gathering all required thing to go ahead. But before going ahead on coding part.
Apple doesn't have any public (developer-accessible) API's that allow what you are trying to do (either sending a SMS or a voice mail response back to somebody saying "I'm not available" or "I will be available in 2 hours").
Nor would Apple likely allow an app like this onto the app store if you wanted to sell or make it available to other people aside from your personal device, as apps that interface with phone functionality may violate some countries' privacy laws, not to mention Apple's own privacy guidelines.