When your hands aren't entirely free, you can dial a phone number by voice on your iPhone (iPhone 4 or 3GS).
Hold Down the Home Button
Hold down the Home button for 3 seconds. (If you have the iPhone earphones plugged-in, squeeze and hold the center button on the earphones cord.)
Speak a Phone Number or Name
The Voice Control screen will open. Now, you can say "Dial 555-1234" or "Call 555-1234". Or, you can speak names that are in Contacts on your iPhone, "Dial Shelley" or "Dial Shelley mobile" (if you have more than one phone number for Shelley).
Works even when iPhone is Sleeping
You can bring up Voice Control dialing even when your iPhone is asleep and locked. You don't have to wake it up. Just hold down the Home button for 3 seconds and talk.