There are a lot of third party companies that will use all kinds of underhanded loopholes to bill stuff to your account. Any time a website asks for your phone number do NOT use your mobile number. Get a Google voice account, and use that number instead.
Also, if at any point you've given your mobile number on an account registration CHECK THE TERMS AND CONDITIONS before accepting them. Many companies will have it in there that you are also agreeing to a charge on your bill even if you have no idea what that charge is.
And call customer service if all else fails. AT&T has a free service
called Purchase Blocker
that prevents ALL charges from appearing on your bill. Even legitimate ones. It's a pain if you regularly purchase things like Ringback Tones or anything like that, but most of us probably don't pay for anything we don't have to do we?