I have been on the Do Not Call List for years and still get these annoying calls several times each day. I do not answer calls from unknowns on Caller ID, ever!
Just got a call, MSKCC on the caller ID, but I didn't answer it and they didn't leave a message on the machine. I'm in Iowa and I would say less than 5% of our calls are legit. Most are related to the presidential race or charity solicitations, so my guess is that if MSKCC isn't Memorial Sloan-Kettering Cancer Center, it's someone fraudulently representing them ... or a new candidate. I stored the number in my phone's phone book with no ring associated. One less annoyance.
The government has done all they can with the Do Not Call list--if charities or political organizations are not allowed to call, how would they get donations? Direct mail doesn't work too well either so the phone is what they have to use. Answer and ask them not to call and most (not all) will respect that request.
These call ARE "legit". Charities and political organizations ARE allowed to call you. Is it too hard just to answer and say 'take me off your list?'-- Try it.