If you have Visual Studio installed, you’ll probably have the Storyboarding add-in as well. You can find it on the ribbon inside PowerPoint, or you can start PowerPoint Storyboard directly from the start menu.
Once PowerPoint is started, it’ll show you the Storyboard shapes panel. This panel contains a whole lot of shapes you can use creating your storyboards or sketches. Not only can this be used for Windows Phone applications, but shapes for Windows Store apps, websites and Windows desktop applications are also included.
The default list of shapes contain most of the common used controls and parts of applications like textboxes and buttons. It also contains more complex controls like maps.
If you need more shapes, or would like to create storyboards for other applications like IPhone apps or just would like a sketchy style, you can download more shapes at the Visual Studio gallery.