I pick it up... wait till some one answers and make them wait for a couple minutes then hang up. I know that some of those dialing things with the call centers are set that they will continue to call until they get an answer on the line 2-3 times. If I remember correctly they can only call so many times in a month, but some of those programs are set up to only count it if someone answers.
those years of working in a call center... the info you gather!