Not precisely "certified mail," but just "mail." They can choose to use certified mail if they want, but it's not required. You should always use certified mail to communicate with a debt collector, no matter if the debt is legitimate or not.
I have done that twice for family members, along with asking for proof of the debt, and that they are authorized to collect it. Both times, never, ever heard from the collector again.
My understanding with debt collectors, is that they usually purchase a spreadsheet with very limited data, and there is almost never a paper trail for said debt. Which also means debt has a habit of showing up on multiple spreadsheets that keep getting resold. I think planet money did a podcast on this topic.
This seems to the the correct understanding. See these two articles that give the view from people who tracked down the debt collections and the bad data in those spreadsheets.
