Did you buy your IPhone from a company you don't want to be related anymore? Do you want to sell your IPhone but customers don't want to use the same mobile network company that sold you the phone? Then, what you need to do is override legally (obviously) its carrier restrictions.
Here is a video that teaches you how to do it in just a few steps. What is portrayed in this tutorial may be done in any computer. All you need is Internet access and ITunes.
Act like you know.