Unfortunately I don't have the luxury of declining an "Unknown Number". Correctional complexes do not identify the number as only the inmate can call out and no call back number is provided.
However I have not received any spam from "Unknown Number" due to this herd immunity to unknown number spam, which I am grateful for.
Given my area codes are no longer where I live, I just assume numbers close to mine are accidents and don't bother answering them. Voicemail seems to validate this when a message is left.
I think that for the most part, people aren't talking about when the phone literally says "unknown number", but when they've received a call from a number similar to their own that they don't recognize.
However I have not received any spam from "Unknown Number" due to this herd immunity to unknown number spam, which I am grateful for.
Given my area codes are no longer where I live, I just assume numbers close to mine are accidents and don't bother answering them. Voicemail seems to validate this when a message is left.