How much will it cost to activate an iPhone 4S To sprint?

So i have an iphone 4S that my sister gave me for my birthday. I had it carrier unlocked when it was in ios 6.1.3 For i can use T-Mobile for 60 dollars. I made a mistake by updating my phone to iOS which i wasn't supposed and i lost my carrier unlock. So now my sim card won't work. Since my iPhone is orginally locked with sprint, i decided i want to switch to sprint and pay 80$ a month for unlimited data/calling/texting etc. I heard sprint doesn't use sim cards. So i'm wondering how do they activate the phone? Can they activate the phone? If so how? And how much will it cost to activate it?

What carrier is the phone originally locked to? Is it a CDMA iPhone 4S? If then you can get the phone flashed to Sprint. If the phone's only GSM then you can't use the phone on Sprint.

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