Where Can I Find A Snail-mail Address For The Vp Of Customer Service?

I have been in ATT-incompetence hell since I canceled my U-Verse TV but not my internet service in Jan.  The cancelation got "Stuck" - their technical term -- and they kept billing me.  I refused to pay for service that was canceled and they've been leaving dunning voice mails and sending nasty letters.  I've spent hours on the phone and sent them letters but they keep replying only on the phone, insisting that I take their word for it that I owe them money.  The bills consist of dollar amounts titled "new charges" "adjustments" and so forth, but no hint of what these amounts actually cover.  I am tired of talking to Shanti & Rajneesh in Delhi.  Billing support is just as anonymous -- they will not share the paper support for their assertions that the past due account amount is accurate.  My spread sheet says they owe me. I want to send a registered letter to an actual human being.  Who is he/she and where can I find him/her?