If your phone is no contract then it would be locked by Sprint to be used on other carriers. If you contract is over simply contact Sprint and they unlock it for you.
In case you are on contract and still wish to use other carrier then visit jailbreak-me.info and just enter your phone details, it will show you that your device can be unlocked or not. If it can be unlocked the website will also give you link to the software for the process.
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Work on my ship for 100 years or Die..!!