If your phone is carrier locked, it will tell you it is when you go to Settings > General > About > Carrier Lock. If it says No SIM Restrictions, it's not carrier locked. If it says SIM Restrictions, it IS carrier locked.
We can't tell you who it is locked by if it says it is locked. You'll have to contact whoever you bought the phone from and then hope the carrier is willing to work with you. There could be unfulfilled payments on a contract for your phone and if there are, they won't unlock it until you pay off any remaining balance owed. And as you are NOT their customer, they really aren't under any obligation to unlock the phone. Apple cannot help you. Apple isn't a carrier and they have no ability to remove a carrier lock at all.
Honestly, if your phone is carrier locked, I'd return it for refund as you didn't get what you were told you were getting. But that's between you and the seller.