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Send them a DV, just to follow protocol. They'll have to stop collection efforts, until they reply. You have a 30 day window from their initial communication.
They can pull your CR's, as it's deemed AR/PP... Account review/permissible pull.
How often they can do that to you is a grey area, and if they're lowlives, they'll do it often.
You may also want to review the former employer policy/contract towards employee education, and see if there's any type of "escape" clause, as you're no longer their employee (factor here is if it was by your choice, or theirs).
I wouldn't recommend any type of phone activity. The good ones have a way to manipulate the conversation (which is recorded). Snail mail certified return (CMRRR).
It has to be a written DV, per FDCPA 809.
The purpose of the DV is to invoke your legal rights under the FDCPA, which makes the CA provide information that they're collecting a valid debt, that they in fact are legally entitled to (purchase/contract hit)
They don't need to reply to your DV, but under legal guidelines, they can't continue collection activity until they reply. Expect a single sheet printout at best.
Once they reply, you have their full attention, then do a PFD offer (in writing/phone if you're gutsy--after mailing DV/getting reply), but never admit to owning this debt, unless your intention is to pay.
I'm sure Guiness will know, but I do believe they have the right to continue pulling, as they "own" it.
A DV sets in place a cease collection bar, so no they cannot do any more pulls on your CR until they validate.