I don't think there's going to be any way to specifically indicate if a user has accessed (or sent) a voice mail from their desktop here.
If they're using Outlook then this is a UM installation and they are also getting email/fax messages in their inbox - you'd have to have a client running that logged information somewhere central when they opened,created,deleted a voice mail. VMO is a client side only app (i.e. it doesn't have logging or db access back to the server - it connects only to the user's exchange mailstore). That's kind of a tricky problem to solve.
You can get message activity for users off the notification logs (i.e. when we turn lamps on/off for users based on message store activity) which would tell you if user's are getting/listening to messages - but you wont get any kind of detailed counts or be able to prove they're listening to messages (i.e. as opposed to deleting them or having a rule move them somewhere or whatever) - only that VMs were going into the store and leaving or becoming read. The message activity report isn't something that can be run for large groups of users - it's intended for troubleshooting issues with notification triggers so it's a "one user at a time" kind of report. Probably not what you're looking for.
There may be some sort of 3rd party tools to specifically log/report on messages based on message class/type information but I've never run across such a beast...
