They called again today, June 17, 2014, at 9:51 am. It was blocked by our call blocker because they called last month. The caller ID was still UNKNOWN NAME. As I wrote then, when I called back I learned that it is the March of Dimes. Perhaps they don't identify themselves because they think few would answer if they knew who they were.
They called again today at 1:48 pm, and the ID was still UNKNOWN NAME. I previously wrote that the number would be added to our call blocker, and it was. I highly recommend getting such a device to avoid these annoying calls.