I've not had this with 8.6.2, but when I experienced this with earlier versions of CUCM, a restart of Tomcat sorted it. (Note that Tomcat does take a while to shut down and get going) Restarting Tomcat shouldn't affect call processing.
If that doesn't fix it, you may have to reboot the server.
(I also see this if CUCM is running in a VM and there's too little memory for the VM)
I'm not able to access my old voice mail messages all of a sudden. The recording says something like 'the message is currently not available'. This has never happened before in all the years I have been using this system. I have t...