The withheld number thing really winds me up as I can think of several organisations with whom I have regular contact whose phone numbers are withheld. Blocking them would be rather counter-productive. If I'm expecting a call, I answer it but if not I let the machine take care of it.
I registered with ... damn, can't remember what it's called but it's the thing that takes your number off the junk calls list, and ensure that my details are kept off the edited electoral roll register.
We do it for confidentiality reasons
If we phone a number and there is no answer then the fact it was a call from a Hospital or particular clinic identifies a fact that the patient may not wish to have known
It is the same reason that appointment envelopes are plain and not emblazoned with logos etc