One reasonably effective defense: "Okay, let me call you right back." Yes, there's always the whole "my phone is dead, I borrowed someone else's" or "I'm calling from a jail payphone", so I think it might become common practice to start making authentication phrases or "tell me something only we know". Another pillar of basic trust that's being eroded on an industrial scale. Sigh.
I mostly answer unknown calls with monotone "hello" and then wait for their introduction before talking normally.
As 99% of the time these are spam calls, I used to respond with something like "I'm fine, but who are you / do I know you?", but that was pretty much always inefficient as that might say their name (which from a spammer is useless information), maybe a sales pitch "how much do you spend on x?" or maybe something deliberately misleading about their company and saying something like even though they're some independent sales crowd getting commission selling contracts for them.
Eventually I found that the most effective response is "Sorry. Where are you calling from and what is this in regard to?" which I've found without fail seems to surprise, disarm them and immediately elicit whether the call is a waste of my time. At which point I either become very friendly (because it's a call I'm expecting) or I simply respond with "Sorry, not interested, goodbye." and immediately put down the phone.
I just want the disruption to be as minimal as possible and to not let myself even get an emotional reaction from it, so I don't want to get annoyed at them, never mind wasting time telling them off, besides, I suspect that my ruthlessly efficient getting rid of them without them even having a chance to try their pitch is received as a super cold shoulder, akin to being told to f-off.