I can't find the follow up letter that I had found on a site. But basically, the law is:
If a debt collector has failed to respond to your debt validation request, then they have essentially admitted, per Texas Finance Code 392.202(b)(2), that the debt in question is inaccurate.
As such, all collection efforts on the debt must cease and any derogatory listing should be permanently removed from any consumer credit reports.
The debt collector has five (5) days after admission of inaccuracy of this debt to cease collections and remove credit bureau listings. Failure by a collection agency to meet this five (5) day deadline means that they have violated Texas Law and would subject the third party debt collector to civil and criminal penalties.
So, I would send a second letter informing them of above. And if they still do not comply, you would be forced to file against them. All of my collections folded from dv. So I never had to go that far. Good luck.