So if exchange is your message store, the voicemail only gets held long enough in Unity to get it to the message store. So not sure if you can see the number of VMs that are not deleted, but you can pull a subscriber report in unity and it will show you how large the message box is as well as some other information. It might show you total messages, but this might include voice and email - not just voicemail. Its been a while since I had to do this.