I puzzled over your post for a bit - I'm not entirely sure I understand what's going on but my best guess is you have VM only accounts setup for your VM users seperate from their email accounts and your public distribution lists are sending messages to the non VM accounts.
Unity does not (and cannot) filter messages sent based on the senders/receipients VM or UM license. Exchange is completely in control of the message distribution when we send to a distribution list either from over the phone or the desktop - it does the address flattening on the server side, we don't do that.
So if you're sending a voice mail to a DL and users aren't getting it then the accounts those users are tied to for their VM accounts are most likely not in the DL in question.
The other question is HOW the UM users are sending their voice mail. You seem to indicate they're doing this over the phone. If so, then my original assesment stands. IF they're sending from the desktop then another possibility is in play that the custom MAPI tags Unity adds to make the message show up as a voice mail are being stripped on the client side (i.e. they sent a voice mail as a plain email with WAV attachment).
You can verify the latter by looking at one of the VM mail user's with an Outlook client and seeing if they have these messages in there but they don't show up tagged as IPM.Note.Voice.Unity and such - if that's the case then you need to figure out if you have a virus filter or a desktop configuration that is removing this information on the send.
Rest assured, Unity is not stripping these properties off or limiting the send address list or anything of the sort. The distribution of messages ot a DL is completely in the hands of Exchange so if the users are in the DL with the account their voice mail box is tied to then the next step is to look at the message properties.
Given that a UM user can's send a direct message to a UM user makes me deeply suspicious of how the message is being sent and where they're sending it.
