Can a lawyer charge me for a free consultation?

I called an attorneys office last week, was asking for advice. She said they could schedule me and appointment. I asked her if it would be a free consultation she said yes so I said ok go ahead and schedule it. I went in yesterday, talked to the attorney maybe 20 minutes tops. Left with the same knowledge I already had. Along with alot of questions unanswered and then got a bill for legal advice today. I call the secretary said I am not sure why she charged you I thought it was a free consultation. I said so did I otherwise I would have never went. Then transferred me to someone else who said apparantly she felt she gave me legal advice and that was not part of the consultation. I asked her how they could do that when No one EVER advised me that I would be billed for any of the conversation we had. Can they do this legally? I live in Oklahoma if this is of any help.