Any call that  does not display  a number, we don't answer  on either landline, or mobile ph.      
In either case there is the opportunity to leave  a message  ( most don't ) and my view is anybody who doesnt want me to see their number has no right to expect me to interrupt what Im doing to talk to them .

But it is surprising how many are unaware that their number is not viewable, due to the complexities of telecommunication systems these days.