Collections for a charge that isn't mine

I received a collection letter for $500 owed to DirectTV, but it was not mine, it was for my rental tenant that has since moved out.

I called AT&T, but the agent is confused, and wants me to call their Fraud department, which will not answer the phone, but sends me to a webpage! How can AT&T change the name and address on the bill, never send the bill, then sell it to a collection agency?