This product does require you to have Caller Display or Caller ID enabled (free with BT / £2.25 from Virgin) for it to work.
You can additionally program it to block Withheld nos., International, area codes, single numbers, but it is preloaded with 200 known problem numbers etc. so you can just plug it in.
Anyone who does get through, who you want to block, you just press the "block call" button on the device, before you pick up the call or during the call. Simples.
