Billing help for a business account

We recently switched our internet provider to ATT, had a firewall installed and signed up for Office@Hand. After a few months we received a notice that we were delinquent in our bill which we had never received and after lengthy waits on the telephone, we were able to speak with a representative who stated the amount that we owed and accepted payment in full and we set up autopay for the account. We just received another invoice stating that we have a past due amount of $550.06 with no explanation. I have tried going online with Business Direct ATT but have been unsuccessful. I am trying to avoid wasting another several hours on the phone to address this. Where can we go to resolve this issue? I am not even sure what the category for this invoice is-internet or phone.