iPhone User’s Guide

Before you can use any of iPhone’s features, you must activate iPhone by signing up for a service plan with the iPhone service carrier in your area and registering iPhone with the network. If you already have a wireless account with the carrier, you may be able to upgrade your account to work with iPhone, or keep using your old phone and add a new line for iPhone. (Some accounts may not be upgradable.) You may also be able to transfer your current phone number to iPhone, or get a new one. [via]

http://manuals.info.apple.com/en/iPhone_User_...