Consider getting a call blocking device or phone.. You can google them and many are around $50.00 or so. 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, not just to the DNC alone:

https://www.ftccomplaintassistant.gov/#&panel1-1
https://esupport.fcc.gov/ccmsforms/form1088.action
https://complaints.donotcall.gov/complaint/complaintcheck.aspx?panel=2

By State DNC lists:
http://800notes.com/res/DoNotCall.aspx

FCC Abandoned Calls Complaint form:
http://www.fcc.gov/cgb/consumerfacts/1088D-R.pdf
email:  fccinfo@fcc.gov

This number called my cellphone twice this evening.  I did not answer the first time, and no message was left.  The second time I answered.  A person who identified himself as with the World Wildlife Fund responded, saying that this was not a solicitation call but just a call to thank me for my support of the World Wildlife Fund.  He was pleasant and courteous, said to have a good evening and that was that.  (I contribute a modest amount each year to this organization.)

None of what you say to do works. Blocking does not work because all these people do is spoof different numbers and call you back. I have been dealing with this for years. They are scammers in India that use programs to spoof U.S. phone numbers to call us. The more you answer the more they call. I stopped answering and just send all calls to block and they still call me. It technically is not a US issue because the do not call list only applies to US telemarketers and collectors. Blocking software in cell phones still shows that you have a missed call from that number which is still annoying. Unfortunately we just have to try to ignore them as hard as that is.

Blocking does work. Get one with area code blocking. As for answering, don't! Pick it up and hang it up as quickly as possible.

And if its a charity, they have to place you on a internal DNC when asked to. If they don't, they can still get into trouble and be reported.

Call block #357.