Thasnks Cazzz, yes I got a call from someone who wouldn't give his name, and persistantly asked for someone who doesn't exist (in my home, at any rate¬) yesterday, but it was also a no. witheld, so hopefully they won't bother me again.
If the number was withheld you won't be able to block it anyway but it is the provider that blocks the calls, not the phone. You may be able to choose to automatically divert to voicemail where the number is unavailable though, depending on what your provider offers for the call divert options.