Your network provider will prob offer a "call barring" service, for a small monthly fee ..... and believe me, it's well worth it!!
Also, people who's numbers are automatically witheld, can still phone you - all they need to do is dial 1470 (not 1471 or 141) before they dial your number and it will "release" their number, i.e. it will not be witheld. Anyone trying to phone us, whilst witholding their number, gets the following message, "sorry your call has not been accepted - anonymous call barring is in operation".