I ran a yum update process on my Elastix PBX, updating all the packages. The update process completed without error; however, voicemail was disabled on all user extensions. Here's how I fixed it.
* SSH into the PBX and cd to /etc/asterisk.
* Find voicemail.conf. If you cat it, you'll notice it's basically empty. But don't have a heart attack just yet.
* You should also see a file called something like voicemail.conf.old_DATE_TIME (e.g. voicemail.conf.old_2015Feb12_183454). The date should match the date you performed the fateful yum update command. If you cat this file, you should see that it contains your proper voicemail user configuration.
* Backup voicemail.conf to be safe.
* Restore the data in the backup file to voicemail.conf. You can copy/paste, cp, etc.
* Make sure the permissions, owner, and group match. They should be 664 with owner and group set to "asterisk." You'll see these same settings on the other voicemail configuration files.
* Apply the configuration. I usually just click somewhere in the Elastix web GUI to get the "Apply Config" link to appear and click it.
* Look at a few extensions in the web GUI, and you should see that voicemail is now enabled. Disaster averted!