I found the answer!!!!
Check this out.
New voicemail notification, but no new voicemail - Google Voice Help
The short answer is that the voicemail is in your carrier's voicemail system, not google voice. To fix it, you must go to Google Voice, deactivate the google voice service for the phone (follow the directions that comes up on the deactivation page) then go into your carrier's voicemail service (hope you remember your password!) and delete the message. In my case, the message was placed from a Verizon store, where a saleschick was trying to get me to upgrade a phone plan. I guess it was an internal line or something so that G/V didn't intercept it. But anyway, it's gone now.
Hope this helps everyone!