The iphone voice command is mainly possible on iphone 3G and upper. You can give different kinds of commands to you iphone such as “call Alex” for example or “dial (phone number, which must be said slowly), “play music”, “play song by (name of the artist or song)” or “what is the music?”    It’s possible to set commands on many different languages so this feature is quite multicultural. To set a language you must go to Settings > General > International > Voice Control.

Here is the original:
Iphone voice commands