If you're looking to go with Sprint, you're out of luck when it comes to prepaid. Sprint stopped selling prepaid service 1-3 years ago saying it was more expensive than contract service. The closest you'll get to Sprint's service is Virgin Mobile. I have no idea if that phone will work with prepaid or not. Otherwise you're out of luck and have to go with a contract if you want Sprint service.
Sprint refuses to sell me the card. They say my only option is $35 per month contract. Is there a way to switch this phone to a prepaid, charge by the minute, phone? I don't need the camera function, only the phone.

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