Sounds like a scam to me, or your carrier is not very good with their paper work.
This happened to me once, kept getting a bill for an overdue account, I phoned and said this was paid on time, they said just forget about it. The next month the same thing, charged again with interest, they said forget about it. Again, I got another call and they were getting a little snotty about it. I said I have the paid bill right in my hand, they said send it in so we can verify, A long pause...I was saying some very choice words. Once I calmed down I said no way, when and if I happen to be going your way I'll bring it in so you can verify. I actually had to do this before they would leave me alone. The bill was only around $18, but hey, it was the principle of the thing. I was right they were wrong, didn't even get an apology, just a shrug of the shoulders.
Have to watch your long distance ones too, another time my provider kept saying I had made a lengthy call to someone in Alaska, I don't even know anyone in Alaska. 'But this call was made from your number', they were adamant about this. I then asked who was this person I was supposed to have called, they wouldn't tell me, this was the last I heard about it. Don't have a problems with them anymore.
Annette