As root, edit rc.local
vim /etc/rc.local
Add the following and whatever commands you want to start up on system boot:
#!/bin/bash
exit 0
Make it executable:
/var/lib/asterisk/sounds/en/custom
/var/spool/asterisk/voicemail/default
If the person has a name recorded, there will be a file called: greet.wav
If it is an unavailable message, it will be called: unavail.wav