How to unlock a Sprint SIM card?

I recently bought an iPhone 6 plus from craigslist and it's a Sprint phone. I currently have the 5c with Sprint as well. I tried to insert the SIM card from my 5c to the plus but it told me that the SIM card is locked. Does anyone know how I can unlock the SIM card or how I can make this work?

Take it your nearest sprint or apple store.

No need to insert the SIM card and you're saying both the phones are Sprint. If that's the case, it won't say that the SIM is locked. It means the iPhone 6+ is not Sprint, it's locked to another carrier. If you need help with your problem, mail me for more info.

If you're getting a locked SIM message (and you're SURE you're currently with Sprint), that phone you just bought is not a Sprint phone.

You'll need to unlock the phone, not the SIM.