Well, you need the hard drive and the proper voicemail software from Inter-Tel. The mailbox upgrade is either a PAL chip, or a license in later versions (I think)
Axxessory Talk is the older OS/2 and NT based voicemail. There are tons of them still out there.
As far as networking and voicemail goes, they have several servers available, Enterprise Messaging comes to mind. I think in most cases, the voicemail is at the 'main' node, not sure if you could have them on both sides.
I would recommend contacting an Inter-Tel dealer in your area, because that's really the only way to get the software and licensing.

Jeff Moss

Moss Communications
Computer Repair-Networking-Cabling
MBSWWYPBX, JGAE