OK, this is what I do, helps considerably. I have 2 phones, One is the house phone - land line - for emergencies, business calls, number to give out to businesses, the other is my cell, the number which is only given to friends and family. I let the  first one go to messages all the time. My cell I don't answer unless I know who it is...period...that's what answer machines are for.

Wow, they're calling every ten minutes. It must really be important. Too bad they won't leave a name or a number so I can call them back. They just called again. This is unbelievable! And again! This creep is a god-damned psychopath!

I called them back and they just kept me on hold with a message that my call was very important to them and hold on the line for the next available agent.  When I answered before they asked for my mother and said they were from Diabetes Care.
She knows NOTHING about this place.  The woman calling was not very knowledgeable about it either.  She hung up on me when I asked what "Diabetes Care" was.  

Called, went to recorder, they hung up. What I can't understand is that my message says: "Due to a high volume of telemarketing calls we are forced to screen our calls, so if you want to speak to (my name) or (my wife's name), you will have to leave a message and we will call you back."
It's been that way for four months. We never answer, without exception. The message hasn't changed the entire time either. Nobody has had any success contacting us by telephone in at least four months, and still I get 30+ calls every day. Most don't stay on to hear the message, so I know they've called before and already know what it says. Most show up as a number the phone company says is not a valid number (999-999-9999, etc.). Some with area codes like 001..., fake numbers, most show the caller as unavailable, not available or it shows the name of the state the call originated from. And these are just the ones that stay on the line until my recorder beeps for them to talk, the rest don't wait that long and I get no information at all about the caller or where they're calling from.
This goes on all day, every day. Months ago when I used to answer my phone, 90% of these calls hung up on me after I answered. They were still on the line, I could hear them breathing or other voices in the background, but they would never say anything, they just hung up.
I can't figure out any logical reason for anybody to do that - it's just bizarre, but after being told that we don't, and won't answer these calls, and knowing that we can't call them back unless they leave their name and phone number, and getting the same result every time they call after months and months - this is beyond weird. These people are psychopaths!
I can't block them because I don't know what numbers to block, I can't block numbers without caller ID because they have caller ID, it's just a fake number and a fake ID. I'm not going to change numbers because I've had this same number for 30 years and too many people know to reach me at this number.
I'm at the end of my rope. The phone company says they have no way to stop them, or even identify them, other than the call blocking features I mentioned earlier that are useless for this situation.
I may have to take them to court for harassment, negligence, not delivering the services they are contracted to provide, whatever. I'm not a lawyer and I know I'll lose, but maybe they will eventually get tired of sending their lawyers to court and pay attention. I'm afraid it's going to take a lot of complaining from a lot of people before they get off their a***s and do something to stop this crap. So don't just write a note here where nobody sees it, send your complaints and any information you have to everyone you can think of. Phone companies, law enforcement, gov't agencies at all levels, friends and relatives. If we make enough noise it will be addressed. The phone companies could stop it in a heartbeat, they just don't see it as a big enough problem for them to get involved, yet. Let's make some noise and get our phones back!