I get it on mine when I restart the phone etc the only way I've found of clearing them is to call my voicemail and delete the messages. It may also work if you save the voicemail message. I think it happens when I listen to my voicemail but do nothing with the message if I want to keep it for a while but when I delete the actual voicemail messages the notifications stop so it's probably most likely an issue in your voicemail rather than an issue with phone. I hope this helps