There are actually two voice dialers built-in. The first is available if you press the voice dial button on the side of the phone. It has a face icon looking like its speaking. It brings up a Bluetooth Voice Dialer menu on mine at the moment, may be bluetooth only? Someone else may need to chime in here. I only use the second option:
The second is the general Google voice control. Long press the search button in the lower right corner of the phone. A "Speak Now" dialog comes up. You can then say "Call John Smith" or "Dial 555-555-5555". You can also dictate emails and text messages, or have it do lookups on Google, Maps, or Navigation. Check this site for more:
Voice Actions for Android