I had the same thing happen to me about 8 years ago. I was new to the internet and signed up for something that I forget what it was. I was joining places like Ebay, Amazon and Yahoo, plus a bunch of other sites. Somewhere along the line, I bought an 800 number. I never did find out from what site is was and didn't know it until the charge showed up on my credit card bill.
I called my credit card company to find out who the charge was from, which they gave me the contact information. I called them several times without being able to get anybody. It would put me on hold and I never had the patience to wait them out. I forget who, but somebody told me that the phone company had some influence over those 800 number charges, so I called them and got somebody to help me out. I told them the number and who was charging me. They called it and got somebody to answer the phone on the first ring. Imagine that?
The phone company verified that I did indeed sign up for that service online at some website, but I forget which one. I admitted to it since that was true, but told them I didn't want the service and wanted to cancel it. The phone company person told them that and it all went away. I lost out on a little money, allot of frustration and learned a lesson.
Good luck, those things are super easy to join without your knowledge, but very dificult to make go away. Luckily for me the one I joined was regulated by the phone company. You might want to find out who regulates the company that is charging you. Government regulatory offices do have allot of power over those they control, and just threatening to call them will get results most of the time. It works great for the phone company, cable tv and the other utilities.
Good luck,
Eddie