For all the cool kids with both iPhones (or any GSM-based phone) and a Google Voice (previously GrandCentral) account, forward your unanswered calls to Google Voice to receive the voicemail and a transcript in your email account.
On you iPhone (or any GSM-based phone), pull up the keypad and type the following, substituting 1234567890 with your 10-digit Google Voice/GrandCentral account:
Log in to your Google Voice account and turn on Do Not Disturb mode.
That’s it. All unanswered calls to your iPhone will instantly be dropped in to your Google Voice voicemail.
To restore your iPhone to the original visual voicemail settings, pull up the keypad and enter:
Your settings will be restored.