Actually no, Asterisk comes with two voicemail applications, the one everyone uses is Comedian Mail, which is a monolithic “c language” application that predates Asterisk, it is NOT amenable to any changes outside the documented behavior,
http://www.asteriskdocs.org/en/3rd_Edition/asterisk-book-html-chunk/Voicemail_id263499.html
(The other pplication included is MiniVM which never really worked and nobody uses 
)