Listen, folks, if you haven't figured it out by now, "billing errors" are business as usual for the telecom companies. They figure that with enough of these floating around out there, in various amounts, at least a certain percentage (typically a fairly high percentage) are going to get paid without question. Easy profits for the company doing it, and if you pay it and end up fighting it later (you actually have very little leverage in this area, if you want to keep your service), well then they just get to keep your money for free for a while. It's my understanding that the accounting rules which telecom companies operate under are biased in such a way that makes this behavior quite favorable, even if the money itself never gets paid.
Back when I was younger I worked for a company which had huge long-distance phone bills. We had a PBX which monitored all this traffic down to the tenth of a second. The phone companies (there were multiple) were required to send us itemized bills on tape, listing every single phone call in detail. We compared our call record data with theirs, and regularly beat them over the head if there were any major discrepancies. This was a constant and on-going battle, because they might very well fix a "mistake", stay clean and honest for a while, then turn up with a whole new set of "errors" when they thought we weren't paying close enough attention any more.
I even worked with a guy once who had spent years at one of the big guns (a name you would recognize instantly). He said they were told that billing could never and would never be correct, so they spent more time working to build systems for service recovery (to keep customers from defecting) than they ever did fixing the billing problems - even though this was their number one source of complaints. He seemed quite shocked that anyone on the outside like me had such a good idea of how the game was actually played.