Has Anyone Dealt with General Revenue Corp Debt Collection Harassment?

I’ve been receiving constant calls and letters from General Revenue Corp (GRC), and it’s starting to feel like harassment. They’re contacting me about a debt, but their tactics seem aggressive, and I’m not even sure if the debt is legitimate. Has anyone else dealt with them?

I found https://protectionforconsumers.com/stop-general-revenue-corp-debt-collection-harassment/ that talks about stopping their harassment, but I’d love to hear personal experiences.

Have you successfully stopped their calls?
Did they report anything to your credit?
Any tips on how to deal with them legally?
Would appreciate any advice from those who have been through this!