Regarding dealing with withheld numbers, contact your service provider and see if they can help.
We're with Virgin Media and we selected an option to block any withheld numbers, so if anyone tries calling and withholds their number, they hear the message along the lines of, "sorry, your call has not been accepted - anonymous call barring is in operation" and they are cut off, so the phone doesn't even ring.
Any organisations that have phones that automatically withheld their number, can still get through if they wish - they need to dial 1470 (NOT 1471) before dialling our number. This releases the withheld function and allows them to get through.