Airpolice
I do block Withheld numbers through the BT system as I don't need to hear from anyone or any company hiding their number.
Take care with withheld numbers. Calls from our GP's surgery and local hospital show up on the caller display as "withheld"; so if expecting calls from such sources answer the phone - a PITA, but better than not receiving a possible vital call.
When I receive the "we've detected your computers at fault" call, my standard reply is to ask "which one" which normally flummoxes the caller, so much so that on one occasion the caller with the Indian accent told ME to stop wasting HIS time!