US law and debt collection?

So I'm way in over my head in credit card debt. For the last year, I just haven't paid anything. . . I don't have the money to pay anything. They've been calling and calling but I just don't answer the phone. I know at least one of my debts has been sold to a collection agency but I don't think the larger ones have been sold. The largest credit card debt is around 12K for one card with others ranging from 2500-7000. What can they really do to me?