How to intercept Call event in an application ?

CTCallCenter* callCenter = [[]CTCallCenter alloc] init];
[callCenter setCallEventHandler:^(CTCall* aCall) {
    NSLog(@"Call Event:\n %@", [aCall description]);
}];

(Original sources: http://www.jesuisdeveloppeuriphone.com/2012/08/24/coretelephony-how-to-intercept-call-event-in-an-application/ )