Android's voicemail indicator can get stuck. This is very annoying.
The problem seems to be that not all voicemail services send the "set voicmail count to zero" SMS to the handset every time you check messages. With a normal voicemail provider, this should get reset after receiving and then retrieving and clearing a new voicemail.
However, Google Voice doesn't seem to set the voicemail indicator at all. Instead, it shows alerts through the Google Voice apps' notifications in the system notifications bar.
If you don't reset your message indicator to zero (by checking an clearing your messages one last time) before switching from traditional voicemail to Google Voice, the voicemail indicator will never go away*.
A fix
All you really need to do to clear the indicator is to temporarily switch back to your cell carrier voicemail, leave yourself a message, then pick up and delete that message. The carrier voicemail should send the voicemail count reset SMS clearing the indicator.
- First, disable your custom voicemail (e.g. by logging out of Google Voice).
- Unset custom voicemail forwarding. On AT&T, do this by dialing ##004#
- Leave yourself a message from another phone.
- Access your carrier voicemail. For AT&T, dial your own number from the handset. For other carriers, you'll need to look up how to do this.
- Delete the message you left.
- Re-configure your custom voicemail.
The indicator should be gone.
To clear voicemail settings for other carriers, YouMail, another alternative voicemail provider, has directions for unforwarding your voicemail.
* verified up to 11 months...