This same company, First Revenue Assurance keeps calling me, multiple times a day sometimes, for weeks on end, then they stop for a week, and continue again over and over.... Their calls go to Google Voice now and they are blocked, however a record of the calls is kept for me to use.
I did not know what this company was trying to collect by calling me over and over. I have not had Cingular for years, and the claim with them was settled back in 2004, when their 3rd party agent screwed me out of a refund, and I eventually took them to the BBB and settled for the return of my monies spend for the phone I had, and the monthly payment fees.
It looks as if they are still trying to collect on something improperly reported as a debt. Which is fine, as the S.O.L is well over at this point.
I feel your pain, and understand what you are going through. Have you contacted them with a cease letter so they stop calling? You may want to do that, and there are many sites that can guide you on how to do this, and properly mail/serve the letters, and keep legal copies for yourself so you can have a well documented record. It starts by making sure you have the calls documented.
Hope this helps.