Couple of thing here…
First, Unity doesn’t have a full set of tenant services features so providing voice mail to multiple companies like this has certain implications. Users calling in from either company will be able to find each other in the directories (outside callers can be restricted from this but not other subscribers), dial each other’s extensions directly, audio text applications are reachable across both etc… as long as the thought isn’t to provide two separately systems that can be neatly and fully segmented you can do this.
Second, you can’t mix VM and UM licenses on a single Unity box. If you guys are using UM on your box you’ll need to use UM licenses for this other group of folks as well if you want them using the same Unity server.
Third, if you decide to use another Exchange server to store their messages on so they don’t chew into your message store space that server needs to be installed in the same forest in the same routing group as your Unity server – ideally in the same domain but that’s not strictly required. Unity can access mailstores on as many Exchange servers as you want so long as they are in the same routing group (this requirement is enforced since routing groups are usually put in place to deal with bandwidth issues and we don’t want Unity trucking out across a low bandwidth connection to pull messages out of a remote mailstore).
You could, in fact, just give them mailboxes on your Exchange server and just hide them from the address book if you’ve got room for the extra message space needed – this would reduce your hardware needs right down.
