I kept one agent on the line for a few minutes and was eventually called some very insulting swear words. If I recall correctly I was called a "bitchass mother fucker"
I think it might be something silly like confirming the number has a person on the other end. For selling to other spammers. Not sure why they can't do both at once though, so maybe I'm off.
And, obviously, they can save a bit by always having more phones answer than they can handle. This way their employees don't waste any time waiting for a call, instead it's your time that is wasted, what costs them nothing.
See https://news.ycombinator.com/item?id=16871131 and https://news.ycombinator.com/item?id=16873537 for what's actually happening.
I occasionally watch this guy who calls tech-support scammers or IRS scammers and attempts to waste as much of their time as possible. He will often impersonate an old lady using a voice modulator (or he'll implement other personalities) to make his performance more realistic. It can be quite entertaining.