@waldrondigital The article that @lgaetz linked to above does address restoring voicemail. How to: Upgrade FreePBX13 to FreePBX14 on Hyper V - Guide
In short, you have to simply yank the /etc/asterisk/voicemail.conf over from the old system. In my experience the voicemail files themselves (/var/spool/asterisk/voicemail/) were intact which really helps out. Now…I had further issues because I had some legacy configuration values in [general] section of my voicemail.conf and Asterisk bombed on reload until I removed those.
PM me if you get stuck and I’ll do my best to help. I performed no less than SIX upgrade tests before I had it all dialed in for a smooth 90-minute live upgrade to 10.13.66-22. I’m not done dialing in the SNG7 upgrade but will probably make a go of it in a few weekends.
Good luck!