Professional Debt Mediation is harassing me for a debt I don't recognize. What's my next move?

I need some advice on how to handle Professional Debt Mediation. They've been calling me non-stop about an old debt, but here's the thing: I have no idea what it's for. I asked them to send me written proof, which I believe is my right, but they haven't sent anything. They just keep calling.

The calls are getting more aggressive. They're threatening to take legal action and have even left a vague message with a family member who answered my phone once, which I'm pretty sure they're not allowed to do. It feels like they're trying to bully me into paying something that might not even be mine.

I found a guide online that explains they have to validate the debt, but since they haven't, what can I do to make them stop contacting me and my family? Should I send a certified letter demanding they cease all communication? If anyone has been through this with PDM or a similar agency, I'd really appreciate hearing what worked for you.