I have paid my phone full but still apple unlocked my phone

No, Apple didn't lock it you did when you used Mint. Best Buy does not sell true unlocked iPhones, you can choose whichever carrier you want but it will lock to the first carrier you use. Truly unlocked phones are only available directly from Apple. If you are within Best Buy's return period take it back for a refund then buy an unlocked iPhone from Apple.

Nope. Sorry. Apple did not lock your phone. Best Buy does not sell unlocked iPhones. ONLY Apple sells fully unlocked phones. The phones at Best Buy lock to the first carrier you connect to. Apple does NOT lock nor UNLOCK phones. Only carriers can do that. If you wanted a completely unlocked phone, you should have bought it from Apple directly. If you can, return the phone to Best Buy. If you can't, you're locked to whichever carrier you activated with. And Apple can do nothing to help you with that.