How can I show my activity(screen) on top of in-calling screen?

I'm writing an android application which listens the phone calling events. What I want to do is, if there is a call incomes or outgoes, an a开发者_高级运维ctivity shows with my customized info. I want to this screen keeps showing during the calling period. However, if a call is coming, the system UI which shows the contact always appears on top of my activity. I've found some apps already realized this function, but mine can't. :( Is there any way that I can achieve my goal? Any help will be appreciated.