Like any phone voicemail, you run out and it stops taking data. This site just stops at 99 and just brings up the message count for the day and then it's wiped and not going anywhere after that. The data has no place to go. Other people in the past have had this problem with much less in the box. Give it a shot. I think you can go back and look at them from the beginning and delete ten at a time? You may only need to delete like 20 or 30 so it starts saving properly. The little red number denotes 'Unread messages' for 24 hours.
Give this a try... in settings, ""Messages Per Category in MailBox"" set it to 50, go to mailbox, reload the mailbox and see what shows up. Mine has always been set to twenty, maybe yours is set at 10 right now.