I just got a call from this number, so I did a bit of research. More than likely, the organization collecting for the Cancer Fund of America is Associated Community Services. Their web page describes themselves as follows: "Associated Community Services is a leading provider of professional fundraising solutions for nonprofit organizations."
According to a report from a Michigan radio station in Feb. of 2011, this outfit pockets about 80% of contributions to CF of A (http://www.wxyz.com/dpp/news/local_news/inves ... ildren's-groups), which means if you donate $100, maybe $20 of that makes it to the Cancer Fund of America.
By all means, contribute to charity but make sure first you know where your dollars are going. Giving to boiler room telemarketers probably should not be your first choice.