Going to throw this out there because I'm totally stumped. I'm thinking we'll just chalk it up to the big bad Microsoft, but before I do that...
I have a customer who just upgraded to Unity 5.0 an exchange 2007 (UM). When they leave voicemail for other employees the message is sent to her draft folder rather than to the recipient. Yea...WTF is right. I saw this happen 4 times when I was testing. Leave a message for my test phone and it never gets there because it's sitting, happily, in her draft folder in Outlook.
Opening the "draft" and clicking send does nothing. Pretty weird. I thought maybe an Outlook rule was somehow sending it there, but when I checked, she had no rules configured.