None of them, I would say. The informative local calling guide atsays that the normal Hilton Head local calling area is just Hilton Head. Hargray Tel's web site at says that for $10/mo you can get local service plus which lets you call Bluffton, Sun City, Daufuskie and Hardeeville. (Or for $25/mo you get unlimited nation wide LD.)
If you want a Hilton Head VoIP number, I notes that Hargray themselves offer VoIP service, presumably with their own local numbers. Maybe they'd sell it to you.