Don't remember anyone else posting about this. Some things to try:
1. Turning off and back on the phone if you haven't done it
2. Send yourself a voicemail and then listen to it to see if that resets it
3. A hard reset is a final option, but that will put your phone back to how it was when you first bought it and you will need to set it up again. People do say that it automatically brings down all the apps you have downloaded though.