I bought one of
these from Maplin
It's £40 - and saved me the £3 per month anonymous call bar fee.
Block international, Skype and spoof numbers, and also area codes too if you want.
For the odd call that gets through, hit the button and the next time they call,, they get two ringtones and then nothing - you might just hear one click on the phone if you're nearby.
Its worth its weight in gold