Nothing. I tried it several times, in one instance waiting three hours to see if it had taken effect. Luckily the sudden group of calls I was getting have stopped. They were very regular for several hours, so perhaps it was a hiccup in a scammer's automatic dialling system.
If TalkTalk is committed to fighting the scammers, it does seem strange that they are starting to charge for blocking withheld numbers, as there can be very few legitimate reasons for people wishing to phone you anonymously. If anything, withheld numbers should be blocked by default and then TalkTalk could make it a paid option for people to allow them – although I can't imagine why anybody would wish to do that.
@Labrat wrote: ...withheld numbers should be blocked by default and then TalkTalk could make it a paid option for people to allow them...
NHS Hospital Trusts have outgoing calls with number withheld. Withheld numbers are the sender's choice. Withheld numbers will not be blocked by default. That's the recipient's choice.