Incoming no is blank

I am using PhoneStateListener to detect calls
Code that I am using is :

 public class PhoneCallDetector : PhoneStateListener
    {
        Context context;

        public PhoneCallDetector(Context cntxt)
        {
            this.context = cntxt;
        }

        public override void OnCallStateChanged(CallState state, string incomingNumber)
        {
        if (state == CallState.Ringing)
                {
                    Toast.MakeText(context, incomingNumber, ToastLength.Long).Show();
                }
    }
}

In lollipop the Incoming no is showing but in Kitkat or Marshmallow the no is always blank.....