How to unlock a Sprint prepaid phone to use on a family plan?

So I have a family plan with sprint. I'm using a really old HTC. I love my service with sprint! I bought a brand new prepaid sprint phone online, new in box, never opened. Can sprint unlock it so I can use it on my current family plan??? I don't want to change plans, I just want to use the prepaid sprint phone on my current monthly plan

If the phone is bought from Sprint, you can use it with them with prepaid or post paid plans. :)

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