I am trying to unlock my moms old Sprint I phone4

I am trying to unlock my moms old Sprint I phone4 to other carriers so I can switch it to my straight-talk account. I have the 6 digit unlock code that Sprint gave to me in order to do this, but now I'm at a loss. I don't know what to do with it, where do I put it? I have tried to look it up and keep finding different methods, something through i-tunes? I'm not sure if this is how I'm suppose to unlock my phone because it always says something about a sim card and this phone operates without a sim I'm pretty sure. Can anyone tell me what I'm suppose to do with this code and how to unlock this phone?

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