Unlocked Mobile Phone?!

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I am thinking about buying an unlocked mobile phone (Sony Ericsson). It has many of the features that the mobile don't have at Sprint/Nextel (the phone company I am dealing with). For example, it has a high resolution phone, mp3 player and it has a SIM card which could technically be used in other countries. The question that I have is: Would I be able to use it with the Sprint network? Will Sprint be able to register the phone with their network? How do people usually get service with an unlocked phone? (Since most phone companies don't sell unlocked phones). I am a little confused about the issue and I would like some answers before I purchase the phone. I wouldn't want the phone and then not be able to use it with my mobile phone network. Pablo