Charities, along with some Survey's and Political calls, are exempt from the DNC, however, they can still get into trouble for not placing you on a internal DNC when asked, you can also check your states laws in regards. Some states have different laws in regards to the calls, which can differ from the federal but as far as I know, Oregon is the only one that does not allow Charity calls.
Do consider maybe getting a call blocking device or phone.. You can google them and many are around $40.00 or less. If its a cell phone, some already have call blocking, if not, there are free apps on the web that will work for some. You can google this. If you have a smart phone, there is a free app called call control. Iphones appear to have apps also. Another one is called Mr. Number.
Report your callers here if they continue to call and harass:
Robocall... No messages left, no one there, even if I pick up. No way to get off their list without speaking to anyone. Even survey companies are supposed to connect to answered calls a certain percentage of the time, and these people are clearly failing to do that, wasting everybody's time. Judging from 800notes comments, the same people also call from 281-241-5383. I'm about to report them to FTC, attorney general, etc.