What makes you think it doesn't work? There's logically no way to count the number of calls you *don't* receive but the fact that internet phone service has made bulk phone calls almost as cheap as email and the fact that there are over 25,000,000 small businesses in the USA makes me suspect the DNC Registry is working pretty damn well as far as legitimate businesses go. It just doesn't stop criminals any more than a "No Trespassing" sign stops burglars.

Called and didn't leave a message.  That means it's a scam.

Called and left voicemail claiming I had stayed in some hotels recently and they wanted to give me discounted rates for the future. Said I had to call a number and claim my offer for cheaper rooms. Have not stayed in a hotel for 3+ years. Scam!

Voicemail just claimed that they were "the travel department" and that's all.

Scammed.  Called ftc.