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A suggestion would be to write a second DV letter, send it CMRR, to Sage. In the letter, tell them to delete it from your credit report. At this point, the debt is unverified (by you) and you can not honestly say whether it is legit or not.
If it is your debt, are you able to pay it off?
Have you called the hospital directly to ask about the old debt?
It has happened, to me, that medical bills get sent to collections, even though the provider is accepting regular, monthly payments. I never pay the collection agencies. I always pay the provider. Even though it's with a ca, the provider still has the records.
Let us know what happens,