I've seen this behaviour in practice before, but I don't think it really clicked with me until Neil May from PostCTI (who was hosting our penultimate Exchange Unplugged event today) told me how pleased he was with it.
This functionality concerns the "missed call notification" feature of Exchange Unified Messaging - as well as the server telling you that you have a new voicemail, it will also tell you when someone has connected to UM but hung up before leaving a message.
In both cases (ie when someone leaves a message, or if they hang up beforehand), if the server can identify their caller ID as belonging to someone in your contacts, you'll see the voicemail or the missed call notification as if it came from the person themselves (it's actually Microsoft Exchange on behalf of <the caller>, but it primarily shows as if it came from the person directly).
So in this case, if I hit "reply" to the notification, it will send an email to the person that was identified as the source of the message. Cool, yes.
What's nice, though, is that if I have my Out of Office message set, and someone calls me then either leaves a message or hangs up, when the notification lands in my Inbox and appears "From" them, their email address will be sent the Out of Office message I've set.
As it happens, I have a contact entry for my own mobile number, in my Outlook contacts folder, but set with my Hotmail email address. When I call my office extension from the mobile, it identifies the contact as the source of the call, and the return address is the Hotmail one, so the Out of Office message I set on my mailbox will be sent to the Hotmail account, since I had associated the mobile number that called me, with that address.
Neil (who spends a lot of time on the road) said this was one of the most unexpectedly cool parts of Exchange UM - customers who call him up and don't leave a message (but who he's already added to his Outlook contacts), will get the Out of Office message as if they'd sent him email. So the next question they ask him is, "How can I get that for myself??"
Seeing this in reality brings the technology alive in a lot of users' eyes.