Como interceptar o evento de chamada em um aplicativo?

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

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