intercept the incoming calls in iphone programmatically

is it possible to intercept the incoming calls in iphone programmatically?

Basically I was thinkging is it possible that we can make an application that is executing (like music application) and if there is a phone call coming in suddenly, we can filter that incoming calls as per some rule and then, say send SMS to that number, and the application is still continues to run....

I know that incoming call event is the highest priority, that is if some application is running it has to suspend untill call completes!