Charges posted to my account from 10 months ago

I have received correspondence from ATT about charges "claimed" to be late/unpaid on my account from November 2018. To date, I have paid all of my bills with At&t outstanding and standing with the exception of the August Bill in question for 475$ due to the added charge that I have been trying to find out about. Every time I call to speak to a manager or account specialist, I get told that they are not available or the system is down. Is this a scam? Because it sure does sound like one. If it was the case that, I did in fact miss a payment, my phone line would have been suspended during the months in question (November- December 2018), I reviewed my billing statements with my bank accounts and they do not reflect a missing payment and my service had no interruptions. I refuse to pay for something that I have yet to see actual proof of and that has been billed in error. Where can I send a letter in mail to At&t?