Very occasionally you need to get a voicemail or greeting from an old Mitel backup.
Officially you would need to restore the backup and get the message out .
The easier way is to open your backup.tar files and find the VOX files. Then using this software you can convert to WAV
I used software from http://www.acarda.com/vox-wav-converter.html its free for personal use
You will need to tick ALAW if in the UK