Being registered does little or no good whatsoever. I've been registered with three different ones for years, but the calls still kept coming- up to 20 times a day!
I have however found a solution (it works for me anyway.) I bought a cheap 'phone that displays the number that's calling. So if I don't recognise the number, or I'm not expecting a call from my normal circle of people .... I just don't answer it! My opinion is, that if it's someone genuine who wants to speak to me, they'll leave a message on the answer phone anyway so I can call them back.
This has definitely worked, as I now only get the odd one that I don't recognise - maybe 1 or 2 a week - and it gives me a great sense of "gotcha" when I just let it ring! :Lol