Hello there, lets see what we can do. This problem has also come to my phone as well. There are a few simple things it could be. One thing it could be is that if you believe you have deleted a voicemail then ended the call too quickly, the message may not have gotten deleted. Just double check and make sure that it is deleted. If the problem still occurs, I always just reboot my phone, and it goes away.
Hope I helped,
warrior.