I get this phone call every day -- sometimes 4 times a day!!  I do not answer it, because it is not in my "contact" list......they leave NO MESSAGE, but hang up. Who is calling me???

I have gotten a call from this number, on my cell, the past week or so, with no caller id info, and they hang up;  NO message is left;  why are they harassing me? Is this a "robo-call" or computer call, or what? I do not answer calls that are not in my contact list.