Welcome to Android Central! First, review Sprint's unlock policy here: https://www.sprint.com/en/legal/legal-regulatory-and-consumer-resources?ECID=vanity:legal. Second, what carrier would you want to use it on? Older Sprint phones generally won't work on GSM networks like AT&T or T-Mobile, and Verizon is notoriously picky about what phones it will allow on its network.