All you have to do is send them a proof of debt dispute letter (which you can find anywhere online, at least an example) and send it to them certified. Upon proof of receipt they will have to prove the debt valid within 30 days, otherwise according to "the law" they will have to remove it from your credit.
As far as this showing as refreshed every couple of months, well that has to do with the debt being sold for pennies on the dollar all the way down the chain of shit. You have the original creditor that attempts to coolect on the debt, they get sick of it and eventually charge the fucker off. All of the rest that follow do everything and anything to collect even a dollar. Scumbags. Don't pay them it's over. If you made the debt, chalk it up for dumb decisions for not paying in this case AT&T.If you didn't then you need to call your local PD and have them start an investigation into identity theft.
