This should be an easy answer but it seems to elude me. Our Example Administrator account has 100's of messages in it. I tracked it down to some device that continually calls the wrong number then just leaves a beep in the VM(cisconnected device now).
How do I delete all the VM at once for a single mailbox?
I'm assuming you're running Exchange on-box with Unity as voice mail only solution? You can still get to OWA from the Unity server. Or, if Exchange is off-box, you can still get to OWA from on the Exchange server.
Just type in localhost/exchange this should bring up the OWA interface for you.