Sorry to tell you this but Sprint phone use the CDMA technology, and the Nokia phone you want to use is a GSM phone, these are totally different technologys. It would be like trying to use a vhs tape in a dvd player. It would work unlocked or not.
The only Nokia flip phone that Sprint carries is the 3155i and you can only get it on their website. I've been looking on ebay for cheaper prices and I've run into another Nokia, the 6101 that Sprint does not carry but many of these auctions say the phone is compatible with Sprint or any other carrier in the world that supports 850/1800/1900 GSM. I don't know what this actually means. I talked with Sprint customer service and they said the only phones that work under their service are the ones that say "Sprint" on the phone.
If anyone is very very knoweledgable on this subject, please let me know if this Nokia 6101 would in fact be compatible.