You can use Unity and CallManager. Setup Unity as VoiceMail only and then use IMAP to have the voicemails show up in their inboxes.
If you are trying to go directly from CallManager to First Class, it may not work. Callmanager would have to roll the call to a digital line or SIP line, but then you would have some sort MWI problem.
I think your best bet is to use Unity and IMAP to the First Class client.