Activated and VM setup I'm guessing? Have you tried calling your self from another phone and leaving a voice mail listening to it and seeing if it goes away? If that doesn't work try a battery pull. If your device doesn't have a battery to pull Google how to simulate one on line. Usually it's something like holding the volume button up while restarting the phone. Holding the volume down will offer you options like factory resetting the device this isn't what you want.
However if you mean email. Try uninstalling the updates for the stock app and installing them again, or uninstalling the app for 3rd party and reinstalling it. There may have been an issue with the install.