I was getting a lot of calls from them too ... and when I answered, no one was there. I called the link above 415-613-4921 and the gentleman who answered was very pleasant and apologetic. He removed my name from the call list (they were calling for a well-known non-profit organization) andtook great pains to reassure me that they weren't some horrid telemarketer!
I, too, have had almost daily calls from this number for the past few weeks. They never leave a message. I make it a habit not to answer any calls from a phone number that I do not recognize. Like many others, I find the frequent calls extremely annoying, but understand that because they're in California, and I live in the same state, they're not subject to Do Not Call policies. Darn!!
I get calls from other distant area codes, and nobody ever leaves a message. Wish there were some way to stop this practice!