I actually called back once and found the group responsible for this is MDS Communications.
They are telemarketers that mainly deal with political and non-profit groups. They have many different numbers that they will use to solicit funds. As telemarketers go, they seem to return larger shares of the raised money to the organizations they represent.
That said, if you want to support a group, do it directly and not through the telemarketers.
I received a call from this number. My Caller ID showed Annville, PA. I didn't answer because I have been getting a lot of nuisance calls from that area code lately. I was surprised when I saw that there was a voice mail a short time later. When I played it back I could hear a lot of talk going on that sounded like a call center. I could hear the caller sigh as he must have been upset that the system didn't hang up soon enough before my answering machine did what it was supposed to.
Shut off the machine and let it ring without answering or try blocking the number. If they're still getting any kind of an answer, they're going to keep calling. After enough times of calling your number, they probably will give up if they get nothing. Consider a call blocking device, too.