Asterisk can be a stand alone voicemail system. You can store voicemail messages in the file system or in database. I have heard of asterisk voicemail systems with up to 1000 mailboxes and 30K messages in database.
You might receive incoming calls directly to asterisk and not use toshiba at all. Asterisk will give you IVR options and extensions. Or might get incoming calls to toshiba and send voicemail to asterisk over SIP or T1. I am not familiar with that toshiba system.
It will not be hard for you to configure asterisk if you are familiar with linux.