There are a variety of approaches: computers autodialling (whether a list of numbers or generated numbers); “mugs lists” that are traded (value changes from “a number”, “a valid number”, “a valid number that answers”, “a valid number that that presses 1 when requested” and “a valid number who has been a mug before”! The last two are the most dangerous: mugs do get mugged again and pressing 1 could actually be initiating a call which could be a premium rate number! If I’m bored, I take the call for entertainment - I still have a couple of dvla fines to pay and the police are nearby ready to take me off to clink for cocking up my tax return!