A DV at this point would likely not be useful.
A DV has a timeliness requirement, and imposes no period for or requirement to respond or send validation.
More specifically, a DV must be sent within 30 days after receipt of their dunning notice to be timely.
The effect of a timely DV is to impose a cease collection bar, thus providing the consumer with a reprieve from continued collection activities until such time as the debt collector first sends validation. They can choose not to send validation, and cease further active collection, such as calls to the consumer.
If a DV is not now timely, they can choose to ignore it and continue business as usual.
If the DV is timely, they can choose not to respond and accept cessation of collection activities.
A dispute relates to the accuracy of reported information, and is available if the debt collector has reported a collection.
It requires the identification of a specific inaccuracy, and a response period of 30'ish days.
That was, in your situation, an appropriate process, but a dispute of a debt or account not mine defense is often not resolved via a dispute.
The furnisher (debt collector, in this situation) is required to conduct a reasonable investigation. If they conclude that the reporting is accurate, they can verify.
That they have done.
There is no administrative appeal under the FCRA from a finding in a dispute.
You can, if you disagree, bring civil action on the basis that they did not conduct a reasonable investigation, and get the matter reviewed on the merits by a court.
Due to the limitations of the dispute process, with verification by the furnisher concluding a dispute, the FCRA offers an alternate process if the consumer asserts that the alleged delinquency or derog is on a debt that was never authorized by the consumer.
An account/debt not mine assertion can be pursued via the identity theft process outlined in FCRA 605B without any need for or input by the furnisher.
If the consumer puts their assertion into the form of a sworn statement before a law enforcement agency, secion 605B then permits the information to be blocked from the consumer's credit report, and thus removed from scoring, without any need to "prove" the negative.
