All other calls should go to voicemail by default.
The way Apple allows you to interact with CallKit makes this hard to implement because a blacklist of numbers is required and a blacklist of all numbers is huge.
I still want notifications for other things (especially text).
I already don’t answer numbers not in contacts (I think this is true of most people) so no real change in behavior.
I tell this to everyone I care for as well, if I don't pick up by the third ring, hang up and dial again. (This is to skip it going to voicemail and also differentiate the behavior).
I don't expect scammers to pick up this tactic because it would double their OpEx and also make it more likely that they are targeted for actually trying to call your number with intent, rather than in error.
Except my missed call log is full of back-to-back calls from random phone numbers in my prefix. Either half the folks in Metro NYC (where I haven't lived in over a decade) are calling me to report the other half got hit by a car, or this adaption has already been made.
The practice of leaving messages directly on voicemail bypassing the ability for the recipient to answer is commonly referred to as "voicedrops". I know this because a client asked for my compamy to implememt this ability, using carriers API first and successive calls as a fallback. Current regulations in the US don't explictly define this the same as a call, IIRC Canada does.
They have (had? still on 10.3 which it claims is up to date) that option under the "Repeated Calls" clicky box -- allow calls if they call back within 3 minutes.
Had to turn even that off since the spammers got smart and were getting through.