No one's called the FBI? (I believe there may be certain phone laws that he could be breaking and asking the FBI to get involved might help you better).

I suppose, if you're system is digital, that it could be used to send data around but I would think it would be more likely, if he's trying to compromise voicemail, that he's either doing "industrial espionage" (trying to find trade secrets) or trying to find unused voicemail boxes so that messages can be saved/storage for others to hear/call into.

To find out where it originates would probably mean a call to your phone provider and/or your voicemail box provider.

I have to admit being a bit neophyte for this area so these are "educated" guesses I'm making. I'm also moving this to Misc Security. Seems a little more appropriate there.