Android: Is it possible to add functionality to the call screen?

Is it possibl开发者_JAVA百科e to customize the call screen in Android? Example would be, two users have the same application, user calls, but the other user cannot pick up, so they respond with some type of predetermined image baked into the application.

Is this possible? Does this break any rules?

The built-in Phone application is open source, so you can clone it and modify it to your own needs. This will let you make/receive calls using your custom UI (I believe you can receive. It looks like there's ACTION_ANSWER that you can listen to and handle). Of course, the user will be prompted on what application to use for theses actions and you'll have to convince your users that they should use your app instead of the built-in one.