I am confused by the whole situation, however. Is your company still paying the bll for services not received? If they are, there's little incentive for anyone to remove the charge. If not, the account would quickly become delinquent and come to the attention of someone. But your description doesn't sound like AT&T is disputing you and claiming the equipment/ service is still in place; rather, nobody is even talking to you.
Also, this sounds more like a accounting issue rather than technical, and thus would be more appropriately handled by the accounts receivable unit of your company. They have more experience dealing with this sort of thing. In every organization I know of the technical people do not get involved in accounting issues other than to merely advise the start and stop dates of vendor services.
Anyway, presumably their billing statement contains an address for inquiries, not just a payment return. I would send a Certified Letter to that address explaining the situation and advising your company will not pay for the improperly billed service. I would not pay for that service.
Because Certified Letters must be signed for, they tend to attract more attention by the recipient. It also provides you with a written record.