I saved an email from a long time ago about what to do with with these automated calls. Don't know if it really works but I guess it is worth a try.
What you can do after answering, if you notice there is no one there, is to immediately start hitting your # button on the phone, 6 or 7 times, as quickly as possible This confuses the machine that dialed the call and it kicks your number out of their system. Gosh, what a shame not to have your name in their system any longer !!!
I answered pressed 9 for details and they said i won a 13 day and 12 night vacation in flordia. Went thru the whole thing until it came to the point where the wanted my visa number. When I asked for verification as to who they were, before freely giving out my info they refused to answer and hung up!!!!