No clue who it is. Probably some scam like most numbers. You can download and app called Mr. Number to hang up on calls or send to voicemail. It works great!
Just got a call from this number (515) 700-0000. I did not answer. I rarely answer numbers I don't recognize or numbers that are blocked. If its important they can leave a message and I call back. And I don't call back unless they leave a message. If I don't recognize the number I go to the internet and at least try to find out where the area code is from. Other then this I just ignore them.