Just today I got two phone calls that showed up as "Unknown". Neither was spam. One was my family doctor, the other a local bicycle shop. If it matters, I'm in Toronto, using an Android phone, with Rogers as my carrier. The first might be due to privacy (of the doctor). The second I can't explain. But in any case, blocking all unknown calls would not be in my interest at present. I can hang up pretty quickly when I hear the boiler-room background and some dude asking me about Air Duct Cleaning.