The answer is registered mail, return receipt requested, and a complaint with the CFPB, although how on-the-ball that agency will be since the new administrator was appointed is YMMV.
I'd be more inclined to write a pro forma letter, then sue for statutory damages for technical violations under the FCRA and/or FDCPA the instant the established deadlines pass. It puts the burden of proving the validity of the debts on them, and if they ignore you or drag their heels, you win by default.