So no, "keeping them busy" is not a viable solution.
I do know enough to say that a simple North American can cause enough ire in an Indian call center worker, that he will call back 4 times just to say "Fuck you, sir!", and that if, by chance, the same North American call center worker gets you twice in a day, the conversation is quite profane.
It only lasts for maybe 15 seconds.
If you want to hurt them more, press 1 (or whatever) to talk to the operator, and string them along.
But the downside - if you care - is that the person working a job in some hellish call center probably gets dinged, performance wise, for spending 15 minutes on a go-nowhere call with you.
But on the gripping hand, you can't hurt the top without hurting everyone along the path - from the person on the phone, up to the call center manager, up to the call center owner, up to the actual business contracting through them.
The Verizon customer service rep suggested "just blocking the number" after expressing shock and awe that robocalls happen, and he had never heard of such a thing!
I suggest at least leaving the robocall running until it hangs up. Any given organization has to have a finite number of outgoing lines, and IVR/VRU's, so keeping them occupied should be a priority for all of us. If you've got the time to fuck around with a Real Human, do that, too. They definitely have a large, but finite, supply of call center workers. Keeping them unhappy should be a priority for all of us. The faster someone quits out of despair, the more that they have to pay to train a new worker.