AT&T charged me like $140 for DSL after I moved out an apartment and transferred the address. I still can't add another phone to my AT&T acct because of the outstanding charge, but hilariously, I can't even pay it if I wanted to because it's too old.
Every once in a while, I'll see that someone new bought the debt:
"Mr. mk, this will affect your credit rating."
"LOL I don't care! Bye!"
Even when the debt is legit, I think there's an argument for once the debt has been sold to a collector, it has been settled. The person you owe got what they are going to get, and you don't owe the collector for providing a service. I know that's not legally the case, but when the bill is small and it's been sold to a collector, who cares? It's not like the collector is going to fix your credit rating upon payment anyway. They'll likely just resell it as if you didn't pay it.