ShepherdsDog ShepherdsDog:
advertisers often work off sequential number lists, 555-1234, 1235,1236, 1237.....(some numbers aren't even hooked up yet), so they make sure they can nail everyone. Each salesperson has a page or two of these number lists per shift and they keep dialing away until they get someone or get hung up on, so the do not call list means shit.
More complicated than that. When I was the network admin for a call center back in 2002-2004 they used a predictive dialer. The computer dials phone numbers based on the predicted call volume of the call center and when a person picks up the phone the call is automatically routed to a call agent. The thing is that the dialer can call more numbers than there are agents available so people end up getting calls and there's no one on the line. Some phone numbers could be subject to as many as six or seven calls in a day. The advantage for the call center is massively increased productivity and the dialing system allows you to manage your staff in a very granular way.