Lenny is a bot which plays a set of pre-recorded voice messages to interact with spammers. You might be surprised just how simple Lenny actually is (though a lot of thought has gone into what he says), yet he’s proven to be very effective at keeping spammers talking for a long time.

Just in the US alone there were over 5 million complaints about unwanted or fraudulent calls in 2016. About 75% of generic fraud-related complaints cite telephone as the initial method of contact. The people behind unwanted calls may be for example fundraising, telemarketing, or simply scamming. The callers may use robocalls playing pre-recorded messages, or real people in call centres. Having real people do the calling makes campaigns more effective. Among the 5 million 2016 complaints, 64% were robocalls, and hence 36% involved human agents. The cost of employing people becomes the limiting factor for fraudsters.

https://blog.acolyer.org/2017/08/28/using-chatbots-against-voicespam-analyzing-lennys-effectiveness/
https://news.ycombinator.com/item?id=15211274