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It doesn't matter if you received a bill or not! Is this for a cell phone? what is your SOL?
Check the CR for the DOFD/DOLA and the drop date.You can use the drop date to get close to your SOL.
When did you last pay on this debt?
Usually a creditor doesn't sell a debt for awhile after it was COd. But once it is sold then you can not pay the OC.
As to your 2nd post that was answered by llecs. She gave you a link to PFD
IMO you should always DV before thinking about a PFD!