Bryan,
The Voicemail is sent as an email to a mailbox (I'm assuming from the context of the question, that mailbox is associated with a Workgroup?) after which it is disconnected.
The email is then picked up as a new incoming interaction and routed to the queue (again, from the context, I'm assuming the same ACD Workgroup?)
Unlike a callback, the two interactions aren't actually related to each other inside IC.
Now, you might be able to mess around to have the Interaction ID added to the email and then read back in as a custom attribute if you need to reference it, but out of the box, I don't believe this happens.
What are you trying to achieve? There may be another method to get the required outcome.