I turned on Silence Unknown Callers on my iPhone 13, but after a day or two it gets turned off

I'm using an iPhone 13 with ios 17.4.1. I keep getting spam calls, so I turned on "Silence Unknown Callers". It does what it's supposed to do for a day or so, but then the calls start coming through. When I check "Silence Unknown Callers", it has been unchecked.

[Re-Titled by Moderator]