NO Caller ID

It's very irritating when you get one of these. 

It's also very irritating when someone has requested an appointment from you and stated that they would like to be contacted by telephone - and guess what, they don't pick up because it comes up as NO Caller ID on their phone. I think we should pick up it might be important, ie, doctors, dentist etc, if it's not, well, tell me what do you do with those unwanted annoying cold callers?