Well, the 'old messages' folder was worth a try.

I noticed the same thing on the attachments--you have a '3 long' message, and my attachments are all 0 k, and they don't open (no surprise given the file length).

However, my voicemail seems to be syncing nicely with vPanel. I will need to 'break' it a bit more to see, for example, what happens when you move messages to different folders and other things like that. But MWI is on whenever there's a message in 'New Messages', and it goes out when I move the message to 'Old Messages'.