I believe that cletus's suggestion is that the short-term inconvenience of picking up on a few such calls will yield the long-term reward of reduced or eliminated robocalls.
The "say nothing" is the important part here I think. Most robocall systems would wait for some kind of voice before starting their spam and that's the point where they probably mark the number as 'legit', not as soon as you pick up as they don't know if it's an actual person that picked up.
It just seems unlikely. I suppose it's not impossible, but in my experience robocalls start immediately. And maybe it would work if you pressed "mute", but without that, you'd leak environmental noises into the call. (People talking around you, cara going by on the street...)
It might just be a trick that people pass along regardless of effectiveness.
