Block unknown numbers

Just a note... to the original poster... you do realize if you block all unknown calls, then ANY CALL that comes to your phone that is not in your contacts should be blocked. Just so you know.
 
Here in Britain, if I receive a call from someone who isn't in my Contacts list, I see the number and choose to take the call or not. If it comes from someone who's masking their number, it shows as Unknown or sometimes Withheld so that's why I gave the above advice.

Maybe countries difffer on this one.