public SomePage()
        {
            InitializeComponent();
            Loaded += Page_Loaded;
            Unloaded += Page_Unloaded;
        }   

        private void Page_Loaded(object sender, RoutedEventArgs e)
        {
            Windows.Phone.UI.Input.HardwareButtons.BackPressed += HardwareButtons_BackPressed;
        }

        private void Page_Unloaded(object sender, RoutedEventArgs e)
        {
            Windows.Phone.UI.Input.HardwareButtons.BackPressed -= HardwareButtons_BackPressed;
        }

        private void HardwareButtons_BackPressed(object sender, Windows.Phone.UI.Input.BackPressedEventArgs e)
        {
            e.Handled = TryGetHandler();//это сам метод обработки, true, если обрабатывается
        }