Can I sue AT&T for them using sleazy and maybe bogus collection agencies?

I feel like I was taken advantage of by AT&T. I had to cancel 4 lines on my account. ( This has been some time ago, maybe 2 years) I was charged for cancellation fees, not too bad since it's in the contract. It ended up being almost 1,000 dollars to cancel my service. It inevitably went to collections. I was hared by them and was threatened by them. They said if I didn't pay then and there that they were pressing charges against me. So I ended up paying them on a credit card. They said the 800 some dollars would settle the debt. Well that showed up on my statement like it should have, but another UNAUTHORIZED amount was also taken out a week later. They took an extra 300 dollars that I didn't authorize and was unable to get in contact with the "Collection Agency" again. I think it was a fraud. AT&T wouldn't let me have that collection agency's number. I finally squeezed it out of a rep when I mentioned "Lawyer" in the sentence. It was a fake number. I don't know what AT&T is running but I'm really upset because...... I was contacted by ANOTHER collection agency about 3 months ago. They said I still owed almost 300 dollars. I was in shock. So I called AT&T and they told me to take it up with the collection agency. So I did, I told the new agency that I paid the balance in full a few years ago. They said if I faxed over the credit card statement then I'd be left alone. I did that, and they still call at least three times a week. So much for that being settled. I can't for the life of me understand why our damn legal system is this messed up that they can't put regulations on collection agencies or on cell phone providers. Any advice would be appreciated, I'm only thinking of suing AT&T because 1- I can't get hold of the former collection agency, which called themselves CCA or "Collection Company of America" to try and resolve this and 2- because AT&T hires these collection agencies to represent AT&T when they collect. In my opinion, AT&T is fully responsible for what happens to people's accounts and should keep records of every person that's done business with them. When I contacted AT&T just about 2 months ago, they couldn't give me any records of my standings, account, debts, or any other pertinent information. Do I have any grounds?