My wife was getting repeated VM's through the day and when she went to listen to the message, there was only dead air. This was very annoying as her VM box would fill up by the afternoon, and family could not leave messages.
We captured the number and did a reverse lookup with Canada 411.
Turns out it was a medical office fax machine that was trying to send a fax but to her cell phone, rather than her work fax machine.
After contacting the medical office and asking them to delete her cell phone number from their fax machine memory, a few days later, no more empty VM messages. Problem solved.
Hope this helps someone.