Charged for Cable Box Returned 3 years ago

I have read that a few other people are having this same issue.  I received a $400 bill from cox last month apparently for a cable box THAT I RETURNED IN APRIL OF 2011.  I have not had cable with COX since December of 2010 (only internet service).  I obviously did not keep my proof of mailing for 3 years.  I assumed COX would have contacted me much sooner if they did not receive the cable box - not 4 years later!!!!!!  I am now being threatened with service interruption, even though they have opened a "research case" and say they need 2 weeks to research the equipment.  I am ABSOLUTELY not going to pay $400 for a box I returned 3 years ago (not to mention that the box was from 2007 and thus is 7 year old technology that I am sure is not worth $400).  I have tried calling COX twice and was transferred 6 times the first time, but never reached anyone that could help me (beyond opening a research case).  Today, the person trying to transfer me could not get the department she was trying to reach to answer the call.  I don't know if COX just needs money and so is charging customers for old boxes - but it seems extremely suspicious (and inherently unfair) to claim that they did not receive a box back - 3.5 years after service disconnection and 3 years after it was mailed back - as I am sure not many people keep the little UPS mailing slip that long!!  If service gets interrupted I will switch to Century Link and file complaints with BBB, FCC and Arizona Attorney General (consumer fraud) and anyone else I can think of.  I pay for the fastest internet connection and have been a consistently paying and loyal customer since we moved to Arizona in 2007 but I am absolutely shocked and disgusted with this.  If others see this and are having similar issue, I urge you to contact AZ atty general consumer fraud (assuming you live in AZ), if they hear same story from many consumers, they will research and act.