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To provide your Windows Phone application access to the operating system (and with it, to the native applications such as SMS, the Contact List, the Camera, and, oh yes, making a call) Windows Phone 7 has a set of predefined Tasks.
Tasks can be divided into two types: Launchers, which launch an application but do not return a value (e.g., sending a SMS message) and Choosers, which launch an application and do return a value (e.g., fetching a phone number).
This video demonstrates using a simple chooser.
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