Service cancelation

I canceled my landline service through CenturyLink over a year ago. Apparently it had AT&T long distance service with it and I am still getting bills for that directly from AT&T (never did prior to canceling the landline, it always came on the CenturyLink bill) for service that they can't even be providing because the landline no longer exists. I've tried calling and get stuck on hold forever. I've tried returning the bills with lengthy explanations and requesting the account be canceled. They don't even bother to respond to that, they just keep adding new charges and sending new bills. Can't find anyplace on the website to cancel service. Tried to set up an account to see if I can cancel online that way, using the only account ID numbers visible on the bill, and it says that account is invalid. 

Somebody must have an idea how to go about getting them to correct their error and stop billing for service they aren't providing!