Just received a call from this telephone number. No message was left. Anonymous callback reached a recording stating that the survey I was calling about was now finished...blah, blah, blah and blah.
The thing is, a human being answering a call, an answering machine or an auto attendant look the same to an auto/predictive dialer.
I have an Avaya commercial telephone system at home. I can monitor incoming calls while the auto attendant is handling them.
Just about every call, you never hear anything from the calling side. The auto attendant message plays, and, eventually you hear a click followed by the line & system resetting.
The dialer is just looking for Hardware Answer Supervision. If a call is not answered, the dialer sees No Answer Supervision.
If an invalid telephone number is called, the LEC generates those 3 familiar tones followed by a recording of some kind. Those calls return No Answer Supervision, thus are not billed and the number should be dropped from the dialer's database.