I've found that I can set up voicemail only users to receive email notification of new voicemails, but my ININ partner has not been able to tell me if I need Unified Messaging licenses for this functionality. To enable email notification I enter the user's client configuration page (in Interaction Administrator), set voicemail alrting method to e-amil address, and enter their email address.

An ININ rep can correct me if I'm wrong, but I do not believe a UM license would be needed for e-mail notification of a new voicemail message... unless those messages are being delivered to an Exchange/etc. mailbox. Just be sure that a user doesn't tell the system to call their DID number to notify them of a new voicemail message. This logical loop can quickly result in thousands of messages.

What you are getting is notifications, which is a message that you have a voicemail. When it emails out, it is duplicated and in two places - no longer unified. With Unified Messaging, the voicemail resides in a message store that is accessed through the phone or through Email and when deleted, it effectively deletes from both places. You need a UM license for Unified Messaging when using Exchange, Lotus Notes, IMAP, or POP for the message store. Using SMTP or using notifications is only a voicemail-only license.

That's exactly what I needed to know.