Ios CTCallCenter callEventHandler从未被调用

我尝试使用CTCallCenter接收来自iPhone的来电。当应用程序处于活动状态时，我调用设备，但并没有发生任何事情，所以不会调用调用EventHandler

原因是什么

我使用安装在iphone5和XCode7上的IOS9.0beta3

我的代码：

- (void)viewDidLoad {
[super viewDidLoad];

CTCallCenter *callCenter = [[CTCallCenter alloc] init];
callCenter.callEventHandler = ^(CTCall* call)
{
    if (call.callState == CTCallStateDisconnected)
    {
        NSLog(@"Call has been disconnected");
        [self ShowMessage:@"Call has been disconnected"];
    }
    else if (call.callState == CTCallStateConnected)
    {
        NSLog(@"Call has just been connected");
        [self ShowMessage:@"Call has just been connected"];
    }
    else if(call.callState == CTCallStateIncoming)
    {
        NSLog(@"Call is incoming");
        [self ShowMessage:@"Call is incoming"];
    }
    else
    {
        NSLog(@"None of the conditions");
        [self ShowMessage:@"None of the conditions"];
    }
  };
}

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我已经解决了这个问题，CTCallCenter*呼叫中心应该在ViewController中定义为属性：我发现在iOS 9.3.5上没有调用callEventHandler。