http://www.google.com/support/voice/...&answer=115093
Call your Google number from any phone, press the * key during the ringback tone or the greeting, and enter your four-digit PIN.
If you dial your Google number from any phone for which you've selected to connect directly to voicemail in the Phones tab of your Google Voice settings, your other phones won't ring and you'll be connected directly to your Google voicemail where you may need to enter your PIN based on the options you've selected in the Voicemail Access section of your Phones tab settings.