Charges for non-returned equipment

I got charged for a non returned Gateway modem; however, I took it back to a UPS store over a month before it was due.  I have the paperwork to prove I returned it.  I called customer service last week and they said they would put a note to expedite the process of checking on if they had received the equipment.  Since then I have not gotten any information from ATT if they found the equipment and the charges are still on my account.  I have read horror stories about it being a long process where ATT will continue to add late fees and even threaten collection agencies.  What is my best option?  Go into a store to get it resolved or is there a better number to call that skips the lower level customer service agents and actually gets my problems solved?  Thanks.