I recently purchased the Samsung Galaxy SIII from T-Mobile and was unable to clear the icon from the notifications bar. I have my voicemail managed by Google Voice so I first disabled forwarding under Phone > Call Settings > Call Forwarding > Voice Call. I then simply called myself, left myself a message and checked it through my T-Mobile voicemail. After deleting the message, the icon disappeared. I then reverted my forwarding settings back to my Google Voice number.