In addition to the suggestions above, you can use the Spotlight. From the home screen, click the Home button again and it will bring up "Search iPhone". Type in a name and it will pull up the contacts (as well as emails, appointments, text messages, etc.) for that person. Touch their name to dial.
But voice dialing is great too, I have iPhone 4 so no Siri but I dial the same way - press/hold home button, "Voice Control" comes up, wait for the tone, and say "Call Bill Smith on his mobile". Very handy!