Sooo happy that someone did the research and write-up on this. I get a handful of these every week and I've been dying of curiosity. Tried stringing them along but could never figure out the end-game.
Now - can someone answer why I get 10 calls a day with no one on the other end? Either I pick-up and it's empty and the call ends a few seconds later, or they leave a 7 second voicemail of nothing.