It certainly worked for us — after a 4 tedious month wait! That was about 3-4 years ago now.
We were being pestered just about every night from some guy on high in a little tin shed in Bangalore trying to flog HP computers, then another trying flog HP computer support services (no such thing of course)! I am a HP computer owner and it was registered online — BIG mistake! Along came a spider. Presto! And I was in terrible web of nefarious marketers in a far away land. They also spammed my fax, but they were not alone: Chinese tour companies, weight loss pills (as if!), car insurance, Viagra (I prefer oysters) and reams of illegible junk from an obscure coffee merchant in Brazil.
So, from experience it worked for us, and our immediate neighbours, and my Mum too. There are something like 3-4 million numbers on the Do Not Call Register in Australia, so obviously something is working and working well.