I received a call from this number on my cell phone. How can they get your cell phone number when it is private?

Caller speaks Arabic.  Can't understand.

I keep receiving calls from this number. But no name on the caller ID and it only says un known. When I dial back some one is saying something in Arabic which I do not understand at all.

Called me and no sound or answer

This is an Islamic institution, calling people invite to islam religion, I mean to alla & or the hell, then to join isis & persecute non-muslims as they're doing in Syria & middle east overall. I got the call multiple times & of course I do read & write Arabic.