Have you talked to a credit counselor? There might be an organization that's free. The way you describe it, I can't imagine that the original creditor wrote off the debt and then sold it to someone who can really collect. That's not kosher.
We had a situation with AT&T/Cingular. We started with AT&T and they converted to Cingular. OK, we just kept our contract, which still looked like and AT&T contract. They kept bugging us to "convert" to Cingular. Since this involved a contract extension (which I did not want) and new phones (which we did not need), I declined. Several times.
Then a phone broke and it looked like we could use another line, anyway, so I converted, extended and got new phones.
Then what appeared to happen was that AT&T billed us ONCE for a portion of a month, which I ignored (it wasn't due) and we started getting Cingular bills, which I paid, presuming all charges were on the bill (after all, it's just one company). The AT&T charge was NOT on the bill and AT&T didn't send us any more bills. After six months to a year, we got a call from a collection agency about it.
We called Cingular to see what had happened and they explained it just this way. I paid it but with a fair amount of swearing.