[How To?] Set privacy of certain fields in profile data? (do it by the user)

[cafeína]

Hello again guys, I have a question that I hope someone can respond me.

I know that in control panel for each additional custom profile data you can set hide or not except for administrators.
What I want to do is for a specific additional field, that the user have the option to do a field public, like in other forums.
For example, a validation box in which user can set his telephone number to public (in order to show it to other users)

Thanks!