I have received a phone call every day, sometimes twice a day, from a 770 number. The caller never leaves a message. My general rule is that I do not answer the phone unless I know who is calling and if I am not in the mood to speak with the caller, I may not pick up. I am not a "phone person". I am on the phone all day long and by the time I go home, if I hear the phone ring, I want to throw it against the wall.
I will never understand people who love to chit chat on the phone for no apparent reason. If I had my "druthers", I would communicate exclusively by email and by text message.
So word to those with no phone number identification, you might as well give up.