What configurations were done after the install on the machine with Asterisk 13? I have spun up a half dozen servers this week with FPBX 13 / Asterisk 13 and haven't had a single issue with the voicemail.
At this point I'm guessing this is a configuration issue and without looking at the system it will be hard to say. You can jump on the IRC support channel on chat.freenode.org in #freepbx and we can try to help you out more there and in real time with you.