I called on an invoice I received months ago becau...

I called on an invoice I received months ago because it was way after the services were provided and I wasn't sure what it was for. The billing department said the invoice had already been paid and I was set. I didn't think anything of it until I recently got another invoice saying payment was past due. I tried calling. It kept saying all circuits were busy. I tried to find additional contact information online and the website doesn't work. I have no idea what to do or what is going on but I don't want my credit affected by this. Any suggestions???