They called my home phone but left no message, so I blocked my phone number and called back. I got a recording that said "press nine to have your number removed from our calling list" and that was it. I blocked my number, so they had no idea who is calling. Thus it's a sham.
Actually if you had any intelligence you would realize that the system will continue to say that you will be placed on their Do Not Call list even when you call from a blocked number. And yes it's probably a scammer or simply a telemarketer however to say it's a sham because of your theory is simply an intelligent. What's the automatic machine going to say? Oops you have bkocked your number so we cannot put you on our Do Not Call list? No the machine says the same thing every time and then will proceed to put a blocked number on the do-not-call list, it's automated and the input it was given was your blocked number.
Recorded message about a low cost luxury vacation. They gave a call back number but said it too fast for me to grab a pen and write it down. Maybe it was targeted at answering machines?