Just wanted to comment on one thing. Robocallers have no need to call to see if numbers are active or not. it costs them absolutely nothing to dial an inactive number (or an active number either, for that matter), which is really the source of the problem. There is no economic down-side for them.
Anyway, it would be a waste of their effort to worry about whether or not numbers are active and put them on any kind of "list". Their list is just sequential numbers starting with 000-000-0000 and ending with 999-999-9999. Of course, their computer likely calls only targeted area codes and skips non-functioning prefixes but other than that, all numbers are good.