Dealing with relentless calls from Professional Debt Mediation? I need help making it stop.

Has anyone else dealt with a company called Professional Debt Mediation? I've been getting constant calls from them and I'm at my wit's end.

It started a few weeks ago. They call multiple times a day, sometimes as early as 7:30 AM. The person on the line is always aggressive and uses a threatening tone, saying things about lawsuits and wage garnishment if I don't pay immediately. The worst part is they called my workplace yesterday, even though I never gave them that number.

I looked them up and they seem to be a debt collection agency, despite the "mediation" name. I'm not even 100% sure what debt this is about. I feel completely stressed and unsure of my rights. I just want the harassment to end.

Has anyone successfully gotten them to stop? What steps actually work? I found an article online that mentions sending a formal cease and desist letter and requesting debt validation, but I'm not sure if that's enough. Any advice or shared experiences would be really appreciated.