It was mentioned on radio NZ yesterday. Apparently it relies on the person getting the call calling back, and they make money on that call back. So when you call back it keeps you on hold, and say to please wait. So during that time they are making money until the person hangs up. 

 

It kind of reminded me of that loophole with free dialup internet many years ago, where companies were able to make some money form people using their free dialup service.