I quote from the leaflet which has arrived with my adaptor.
"Anonymous caller rejection, Ths means that anyone who withholds their number won't be able to get through to you. Got friends and family with a number that's automatically withheld? No worries. They just need to dial 1470 before they call your number."
I think tha pretty well covers every NHS number and no doubt hundreds of other organisations.
From my experience recently most nuisance callers are now managing to use a land or mobile number to circumvent being automatically rejected.