I got several calls from 202-888-9724 also. I called them back and there is a prompt to remove your number. It's kind of tricky, you have to push 1 (the option to hear more about their organization) then listen to the short message, it will then give you the option to be removed from their calling list. Hope this helps.
The callers are the American Institute For Cancer Research (AICR) one of the very lowest rated of all "charities" because of the fact that most, if not all, of the donated monies go into lining the pockets of the organizers and the paid telemarketers, and not to any cancer related causes. These disgusting scammers take funds away from, and make people suspicious of legitimate charities. Make sure to warn others of these phonies before they are tricked into donating!