spelling messages using siri or dragon dictation

Is it possible to spell out words using either Siri, the dictate button on the on screen keyboard or Dragon dictation? I don't use voice dictation at the moment but that would be useful for example when typing in a web url in safari or just to make sure it understands what I'm saying. It seems to do a good job of recognising numbers but not the name of contacts when I say them for example.