Based on the delays in these sorts of calls, I’d guess that they’re robocalls which dial way more numbers than they have operators for and try to filter out no-answer and voicemail pickups automatically. Then if you pick up, they route you to an operator. If there’s no operator available, it just drops the call rather than reveal which annoying company just wasted your time. That way the operators— clearly the most expensive link in the chain — are always engaged. Just a guess though!

Even if that’s wrong, I’m sure you’re right that they collect caller-pick-up stats. I imagine even cursory vetting would dramatically increase the resale value of their lead list.