Guess I'm not to savy either because I never heard of this before myself. Seems anyone can get through to us no matter what we do. Maybe the call blocking is the best way to avoid these calls, we get so many it's frustrating sometimes.
I know I've come home to find messages on my phone and some are from different call centres who actually leave messages....it seems when the answering machine goes off it activares their machine and it leaves a message automatically.