When creating an application for a phone, keep in mind that the application needs to interact with the phone environment. For instance, if you are building a contact list, you might sometimes import data from an existing contact list in the phone or you might also want to send an SMS or directly call a contact. These operations are interacting with the actual phone operating system from the application.

You can use Launchers and Choosers in your Windows Phone application to enable a user to perform common tasks on the phone environment. The Launcher and Chooser APIs invoke distinct built-in applications that replace the currently running application and when the actual operation is performed, the built-in application is again replaced by the application running on background with a callback, thus giving a seamless experience to the end users.