FWIW, this is what has worked for our family:
-We are all trained to NOT answer the land line unless we recognize the caller;
-The voice mail message starts with a stern "Take this number off your call list";
-After a second or two of silence, it continues with "If you know us personally, then you can leave a message. Thank you."
We went from 10-15 calls/day (never move: your number is sold to EVERYONE ON THE PLANET) to 1-2/month. If someone messes up and picks up the line when they shouldn't, we go back to 3-5 days of incessant calls and then the frequency falls off again.
A number of our friends have started doing this as well, because it's so effective.