I ignore calls which have no caller ID, letting them go to voicemail.
In the UK many moons ago, I had a service on my mobile that simply diverted all such calls to a message that said "The owner of this number has chosen not to accept anonymous phone calls, please call again using caller ID" or something along those lines.
Recently I have been getting a number of calls which are both caller ID withheld and not leaving VM.
Is there a service such as the one I mentioned above available here in NZ?