Usual scammer MO, I have deployed several techniques to reduce the number of junk calls: First, having your home number unlisted and adding “anonymous call rejection” feature stops a significant number of calls. An additional solution which is free and works well is recording  the SIT (Special Information Tones) for call failed on the beginning of my voicemail greeting followed by my regular message. Most, but not all robo-dialers respond to the SIT tones and mark my number as bad and delete it from the call list. Here is the link for the SIT tones:
http://www.scn.org/~bk269/errorbeeps1.wav
I do block the numbers, but this is not very effective, some robo-dialers spoofs a different number each call..
These measures have reduced the junk calls to a few a week, versus several or more per day.