Asterisk is an Open Source PBX and telephony toolkit.

This package includes the standard files-based voicemail storage. This is normally the one you use and is the one that will Just Work. The only reason not to install it is if you want to use a different Asterisk voicemail storage (ODBC or IMAP).

For more information about the Asterisk PBX, have a look at the Asterisk package.