How to unlock a phone?

So I bought an iPhone off a guy for $400 and he told me it was unlocked to use straight talk. Well it's not. The phone is originally at&t so I got on their website to try to do it myself but I need the previous owners info for it. I bought the phone off a barter page and after something's sold it's deleted and I have no way to contact the man. Does anyone know if I can unlock it by calling or if there's something else I can do? I'd hate to have to sell it 😑