Large telemarketing organizations use automated predictive dialing systems to make phone calls. The individual telemarketers do not dial the phones and they do not sit on the phone listening to it ring or listening to busy signals. The time of the telemarketers is valuable to the sponsoring organization, yours is not. They want to have a new live sucker connected to the telemarketer the instant the telemarketer disconnects from the previous sucker.
They have more outgoing lines set up than they have telemarketers. They have statistics about how long the average phone call lasts, what percentage of calls are answered and so on. They attempt to time calls so that you answer at the same time the telemarketer is ready to talk to someone new. As soon as you answer (and they use certain heuristics to make sure you are not an answering machine), your call is switched to the first idle telemarketer.
Doing this exactly is impossible. Some days more people answer the phone than others. Some days people stay on the line with the telemarketer longer than others. The systems can be tweaked. You can set up a more aggressive dialing pattern, but that results in more calls being answered when no telemarketer is available to talk. You can set up a less aggressive calling pattern, but that results in more telemarketers sitting at their work stations waiting for a call. Guess which they prefer?
To summarize, they make a lot more calls than they can answer in order to make sure that no telemarketer is sitting around waiting to talk to someone. If you have the misfortune of answering when no telemarketer is available, you get hung up on and put back in the queue to be called later.
Edit: Just like Senegoid said more succinctly.