Well, you wouldn't have anything close to UM - your lamps and notifications wont work, phone access to messages would be fouled up (i.e. if you move messages you can't get at them over the phone at all, if you copy messages then they will languish on the mailstore until you call in over the phone to delete them) - not a good solution.
The way to go about this is to use the web based inbox - Unity can sent emails as notification mechanisms to any email address - in this case the email will contain a URL to the user's web based Unity inbox which lets them get at their voice mail from their desktop and also keep the read/unread/etc... status on the server and over the phone consistent. So when they get a voice mail an email will show up in your corporate inbox with a link back to the box. Again, it's not unified but it's about as close as you can get in this scenario.
