My solution was to buy a digital phone (Dect 6.0) which has a "do not disturb" function so the phone will not ring and will go automatically to voice message. And it also includes a function that allows you to block unwanted calls so when they call again the call is automatically terminated.
My strategy is to block all the "unknown" callers (I have caller ID) and leave them blocked for several weeks. Eventually they give up when they do not get a ring. Then I clean the phone log so I have room to block the next batch of illegal calls (my model allows 20 blocked numbers, I wish it were 100). I am on the federal do not call list but clearly that is ineffective. Alas, the government can do nothing to help us. So I have to work at it but at least I have some peace from the phone harassment.
Calls ALL the time and my dad won't answer so I did and they hung up. When I tried to call back, all I got was a busy signal which is a GOOD thing on their part. Turned them in to the DO NOT CALL LIST. Irritating jerks!