Many organisations have additional message classes from other bolt-ons to Exchange (such as Fax software and the like). It’s fairly easy to get Enterprise Vault to archive these, as outlined below :
Step 1 – Add the message class to the Directory
Right click your directory in the VAC, go to properties, then the Message Class tab, as shown below.

Click on Add, as indicated, and add your class, and a description. For example add IPM.Voicemail

Step 2 – Update the policy
Now go to the mailbox policy (or policies), and update that:

You need to tick the checkbox.
Bonus Step – Preventing archiving of a...