Calling isn't zero cost, and that spammer time isn't zero cost, so in this case, there is a incentive from the spammer to weed out people who costs the most.

So isn't the popular idea that you should NOT answer spam calls wrong? Logically, you should answer every spam call and try to get them to stay on the line for as long as possible, therefore maximizing their cost.

This is assuming they have some CMS software on the backend that allows them to categorize numbers.