How do I unlock a htc ozone?

I have an locked HTC OZONE I don't know how to unlock it in order to use my at&t sim card. I called the verizon network failed! They told my friend to deactivate her phone and they would activate this new phone I got and the sim should work . That failed miserably. I was wondering if I could take it anywhere for cheap or if there's a way I can do it for free. The guy who sold me this phone told me my at&t sim card should work for that phone but it keeps coming up as invalid. Help what should I do???