I answer with silence. I wait for them to speak first. Not even once has a scammer ever spoken first. I say nothing, they say nothing for a full minute before they hang up.

I mostly just don't answer them unless it seems like something that may be legit.

I think it's a somewhat South African cultural thing, but when I get calls from businesses or spammers, the first thing the caller tends to say is "Hello, how are you?", which is completely stupid when you're calling someone who wouldn't know who you are, so it tends to immediately make me annoyed that they don't know that they should have introduced themselves first.

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 <major brand name> 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.