If you open up the VoiceMail Pro client, you can go to Help > About to see the version number. I am assuming that this is VMPro, as it sounds like the outcalling feature to me. Does his cell phone actually ring, and when he picks up he's able to login to his office mailbox?
I'm not the most familiar with outcalling, but I suppose its possible it will call with old messages if they aren't saved or deleted. The voicemail may still consider the message as unread, so the next time either voicemail is restarted or a new message is left any messages that are not saved or deleted will probably show up as new.