You can tell your iPhone 3GS what you want it to do with your voice. Here’s how.

1. Press and hold the Home button below the screen for 2 seconds until you hear a beep.

2. To call a person, say on of the following:

• Call John Doe
• Call John Doe home
• Call John Doe mobile
• Call 5555555555

To play music, say:

• Play
• Play Music

then you can say:

• Pause
• Next Song
• Previous Song
• Play Green Day
• Genius
• Shuffle

Source: iPhone User Guide