When I was getting robocalls a while back, I started saying "hi" and letting them talk, then I'd put the phone on mute and continue whatever I was doing. The calls stopped a few weeks after that. Usually this would take about 10 seconds of my time, and about 90 seconds of theirs, per call.

I'm pretty sure that over the long term, it costs me less time as it made the problem completely go away.

Things that I'd tried before that, that didn't work, include: immediately hanging up, politely and no-so-politely asking them not to call back.