The homebrew-cask repository has moved. Did you try:
brew tap caskroom/cask
brew install brew-cask
brew cask install virtualbox
instead of phinz?
Update:
As Jon Nalley points out, brew install brew-cask
is now deprecated:
Important December 2015 update: Homebrew-Cask will now be kept up to date together with Homebrew (see #15381 for details). If you haven’t yet, run
brew uninstall --force brew-cask; brew update
to switch to the new system.