You haven't provided enough information to answer the question. In the US, the only source for unlocked iPhones is directly from Apple. If you buy a phone from a retailer such as Best Buy or Walmart, the phone will lock to the carrier used to activate it. It doesn't matter whether you paid for the phone in full, that's the way it works. Only the carrier it is locked to can legally authorize unlocking it.
If the retailer told you it was an 'unlocked' phone, they lied.