Detect phone calls on iOS with CTCallCenter (Swift)

I wanted to try to detect incoming phone calls in my app. I created a new Swift project from scratch just to try some code. The only thing I did was importing CoreTelephony in the ViewController that is created with every new project and I also changed the viewDidLoad() to:

    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

    let callCenter = CTCallCenter()
    NSLog("start")

    callCenter.callEventHandler = {[weak self] (call: CTCall) -> () in

        self?.label.text = call.callState
        NSLog("Call state")
        NSLog(call.callState)

    }

I also tried without the [weak self] since I am new to swift and not sure of what it entails.

When I run my new little app via XCode on my phone nothing happens when a call is received, disconnected or anything else. No error what so ever. Do I have to do something more in order to use the CoreTelephony framework and the CTCallCenter?

Regards Johan