from application background prompt user with a dialog on call button click in Android?

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http://stackoverflow.com – I am working for some background tasks in Android. I have a condition, like my application is running in background and when ever user clicks on phone call button, I have to show a dialog. What I am thinking about, that I have to run a service and waits for the green call button click in background. Is it possible? Please give me some more ideas. (HowTos)