If you are doing this with UM, you can complete this task using Exchange and Active Directory.
In AD, you can customize the permissions given to another AD user on what they can do with a paticular inbox.
Set the voicemail box as a "shared mailbox"
In AD on the exchange Advance properties of this mailbox, give the Manager full access, give the "one person" read only.
At each Outlook client, you will need to configure Outlook to attach to this "shared mailbox" so he or she can view the messages in it.
i don't think you will be able to do this through the TUI interface.
