This was an interesting study. Here are some conclusions:

• To better understand how these unwanted callers operate, we monitored every phone call received to over 66,000 phone lines in our telephone security lab, the Robocall Observatory at North Carolina State University. We received 1.48 million unsolicited phone calls over the course of the study. Some of these calls we answered, while others we let ring. Contrary to popular wisdom, we found that answering calls makes no difference in the number of robocalls received by a phone number.
• Using these techniques, we learned that more than 80% of calls from an average robocalling campaign use fake or short-lived phone numbers to place their unwanted calls. Using these phone numbers, perpetrators deceive their victims and make it much more difficult to identify and prosecute unlawful robocallers.
• No one knows how robocallers interact with their victims and how often they change their strategies. For example, a rising number of robocalls and scammers are now using COVID-19 as a premise to defraud people.

https://news.yahoo.com/does-ignoring-robocalls-them-stop-114409203.html