When switching to another phone company do you have to buy their phones all the time?

Most cell phone companies have restrictions on their phones. Meaning they put a block on them so you wont use it with another company. But theres alot of places you could find on google or yahoo where they teach you how to take off those restrictions (Unblock). Search something like "how to unblock my LG Optimus S" Once unblock you may use it with another company.