It could be either way. Obviously targeted to specific demographics is the most effective, but phone calls especially robocalls are very cheap. Even ratios as l

It could be either way. Obviously targeted to specific demographics is the most effective, but phone calls especially robocalls are very cheap. Even ratios as low as 10^-6 are profitable when you make millions of robocalls. The USA Do Not Call list is a de facto live number list. Illegal telemarketers (redundant?) can directly use it as their target list. Another method is to randomly call numbers and mark "Hello?" responses as live.