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Looks like its time to call his bluff.
He asserts that they sent prior dunning notice. You asert no such notice was received.
Since you assert non-receipt of dunning notice, then any DV you send will, in your opinion, be timely, and thus invoke a cease collection bar.
Thus, I would send a DV request.
If the debt collector then calls or writes without first providing validation, you can assert a violation of their cease collection bar under FDCPA 809(b).
You could either then file a formal complaint with the CFPB or initiate civil action for their violation of section 609(b).
Then would then be forced to assert that your DV was not timely by showing sending of a dunning notice more than 30 days prior to your DV request.