
I don’t know if it’s a universal thing, I’ve never bothered to research further. On my several-year-old Oneplus phone (Android), if I single-press the power button, it mutes the ringer and vibrate but the call doesn’t end or reject (I could still then go and answer or reject the call normally, it doesn’t affect the user interface, just the ringer/vibrate). That’s how I’ve been “rejecting” unknown calls for a long time. A simple, elegant solution that doesn’t give the caller any hints.
That solidifies my suspicion that it’s a standard Android feature… I also don’t get many spam calls, and only distinctly remember performing that action on this most recent phone.
Based on OP’s comment “…I always assume that rejecting the call outright will also be detected as a deliberate action and therefore a person is on the other side…”, I figured maybe they didn’t know about that feature and/or have an iPhone and they somehow don’t behave that way.
I also miss the old days of Android… I got a smartphone specifically to play Pokemon go in 2016 lol, up until that point I was still rocking one of those Casio Gzone indestructible flip-phones. Walked into WalMart, bought the cheapest LG whatever phone I could find (Android 5 I think?), caught a bazillion Pokemon. I remember buying multiple batteries for longer sessions, because you could just pop the back off and replace it on the go.