If you say “Helvetica”, it’s going to use Helvetica. If you say “Arial”, it’s going to use Arial. If that font doesn’t exist, it falls back to native.systemFont.
On iOS 6 and earlier, native.systemFont is Helvetica. On Android, its Droid Sans. On the Kindle Fire and maybe the Nook, it’s Arial. Corona won’t switch font’s mid-string. If it doesn’t know a glyph, it just won’t display it or display a block.