AT&T unleashed a torrent of debt collection calls on my wife when they sold a debt for someone with the same name as hers to Joel Cartis LLC (a typical scam outfit that operates quite illegally). Of course, the debtor couldn't be located so they came after my wife since she lived in the same area codes. Cartis didn't tell us that At&T did this in fact, since Cartis didn't have her SS number (they asked for it!) there wasn't much they could do except threaten. We found the source of our grief by going through all possibilities, based on her having a phone in her name using the wonderful Zaba search engine. There, we found listings for at least two people with her exact name.
When we talked with AT&T, they said- yes, we know it's not you, and too bad these people are calling you. We won't intervene.
Everyone should read and know the FTC regulations concerning fair debt collection practices- especially section 809. Hit these people with it when they call you & fight back.