I believe that I have had this problem in the past, but I can't remember what I did to fix it.
I have a user that while playing a voice mail message, will get disconnected. As in, Unity just hangs up on them. It doesn't seem to be related to any particular one message, or the length of time that the user is on the call. As of now, this is only a single user reporting the problem. I have a feeling just waiting and doing nothing may resolve it, as there are cleanup and repair jobs that run at 1:00AM. But I thought it might be nice to see if anyone else has experienced this and what they did about it.
I am running Cisco Unity 8.6.2.