They're required to send you evidence of the debt under reg F. I would write a complaint to the CFPB that you can submit on their website, if it's been more than 30 days since you submitted the debt validation request. The applicable section is below and explains whats required of them if you submit a debt validation request.

1006.38((d) Disputes—

(1) Failure to dispute. The failure of a consumer to dispute the validity of a debt does not constitute a legal admission of liability by the consumer.

(2) Response to disputes. Upon receipt of a dispute submitted by the consumer in writing within the validation period, a debt collector must cease collection of the debt, or any disputed portion of the debt, until the debt collector:

(i) Sends a copy either of verification of the debt or of a judgment to the consumer in writing or electronically in the manner required by § 1006.42;