I have friends and relatives who withhold their number, would it still block their calls?
No, unless manually program the unit to block all WITHHELD/PRIVATE calls.
In this case relatives, friends and numbers of importance can still dial 1470 (UK) or *82 (U.S.A) to override the WITHHELD/PRIVATE blocking code.
You would need to tell these callers to do this as they may not be aware that you are blocking such calls. So they would basically dial 1470 or * 82 before your actual number and they can then bypass the block for that one call.