The most fascinating thing I found when I searched what you recommended was this news story:
That basically could potentially explain everything that has happened to his phone. His phone book could have gotten hacked into, and then the caller ID spoofing would make his coworkers think that it was him calling, when it wasn't.
This is all really far fetched, I must admit, but it's the only explanation I can think of. DBF really doesn't have any enemies and doesn't give out his phone number to a whole bunch of people. I can sort of understand that his family would assume it would have to be someone close to him, i.e. myself, that would be doing this... but that still wouldn't explain the manly voice on the voicemail. What I'm wondering is if changing his phone number would be good enough to make everything stop, or if it might be necessary to get a new phone, etc.