I've been getting these call for several years.
This what I've found to be pretty effective.
I made a list of spam callers phone numbers and when one of them call, I answer with
AN INDIAN ACCENT AND SAY HELLO, HELLO, HELLO, AND I DO NOT IN GAGE IN CONVERSATION WITH THEM. This drives them crazy and they hang up.
Mission accomplished! Then they put me on their DO NOT CALL LIST. !!!! I win.  

Now, those call have dropped off a lot.
It works for me, Try it You'll like the results.
9/6/13

Got call from this number--no message left. Annoying.

THIS NUMBER CALLED MY CELL, I ANSWERED AND THEY DISCONNECTED.

Please do not call.

I keep receiving calls from this number. If you call back, you just get an answering system that provides no identifying information. If you answer, you get silence and the call is quickly disconnected. Someone wrote that the number apparently belongs to the Leland Lee for SF mayor campaign. I would guess it's more like someone trying to get people angry at that candidate (if it's true that the number is associated with him in the first place). What sane political candidate would campaign using frequent hang-up calls??

According to my VOIP service provider website, this phone number belongs to Leland Yee.  In other words, this number is either calling to get you to vote or volunteer for Leland Lee for SF Mayor.

Same as RE  call and no message.

Just received a call from this number. I don't recognize the number and they didn't leave a message.

they've called once a day for the last four days. who are they?

Calls 3x in one afternoon and leaves no message, calls many times during the week.  I would love to know who is calling and block calls.

Same here. I've received 3-4 calls over the past week.

Ditto. No one on the line after picking up.