ATT & Southwest Credit Systems Fraudulent Credit Collector Plano, Texas

My husband and I cancelled our services with ATT U-verse a year ago this month, as we were moving and their program was not available in our new neighbourhood.  As requested, we paid our last bill and shipped the DVR boxes back, being smart enough to keep tracking numbers on file.
Three months later, we got a bill from ATT for un-returned equipment.  We immediately contacted ATT, provided them with the tracking numbers from the shipment, and were told it was an error on their part and it would be noted on our account as resolved.
Three more months go by, and we get another letter, this time from Southwest Credit Systems for an outstanding balance of $94.28, again for un-returned equiptment.  Fortunately, my husband works in collections at another major phone company, and knows the next step.  He contacted the PUC (Public Utilities Commision) concerning the matter, giving them everything from the tracking numbers (again), to the names and dates of every associate he'd spoken to along the way.  They looked into the matter and were told it was going to be resolved and wiped off our record within one week.
Today we got another letter from Southwest Credit for the same amount, but if we paid within the next ten days, they'd give us 20% off.  My husband has already contacted the PUC and is in the process of faxing our paperwork on.  By Tuesday, they expect a result, as by law, ATT is required to take care of the matter as they are the company claiming the original debt.  If they don't, they end up getting fined.  Hopefully this will be resolved soon as it's going on a year and we've long-since lost our patience concerning the matter...
Best of luck to anyone else encountering this problem.  I am hesitant to believe that Southwest Credit is legit- they may be, but the communication between them and ATT seems to be lacking any integrity.