This is unsupported within iOS.
If your application is active when a call event takes place, the system dispatches the event to your handler immediately. However, call events can also take place while your application is suspended. While it is suspended, your application does not receive call events. When your application resumes the active state, it receives a single call event for each call that changed state—no matter how many state changes the call experienced while your application was suspended. The single call event sent to your handler, upon your application returning to the active state, describes the call’s state at that time.
The best you can do is detect phone calls while your app is open.