I don't know why or how the Facetime application is gone from my iPhone 4. How to install it back?
There is no FaceTime app, it's built into the iOS. Settings>Phone>FaceTime>On.
Right, it's not an app like on iPod touch and iPad. To call someone using Facetime, go to them in your contacts, and it'll have the Facetime icon.
The manual has your answers... http://manuals.info.apple.com/en_US/iPhone_iOS4_User_Guide.pdf
Retrieving data ...