Medicare Scams: Braces

I'm old.  I'm on Medicare. I see ads on Accuweather for knee and back braces covered by Medicare.  I get phone calls offering free braces. 

When I google "Medicare scam braces" I get a long list of hits.  Apparently some scams can work by getting access to your Medicare number, then billing Medicare for stuff, which may or may not be delivered, but never is prescribed by a doctor.