file a dispute with the credit reporting agencies (all 3 if it is listed with all 3)

the old account is long beyond the reportable time. The new account, what I read, has a 0 balance so there is nothing to collect on. If you can get a written statement from Sprint with what you said they said, it would be good support for your argument.

Personally, I would not contact the collection agency at this point in the issue. There is no reason you should have to.