Modern phones could do with an "FO" button for an entirely different purpose. Don't like the current call... hit the FO button... let them know. In particular a disturbing practice I noticed in Australia. The bank calls, I pick up and a recording: "this is XYZ bank, please hold an operator will be with you shortly". A golden opportunity for an FO button. It could work like an email spam button Enough FO's and the number gets auto blocked.
It already exists on many telephone service providers in some countries. It doesn't have a specific button, and it isn't standardized across providers. But on one provider in the United Kingdom, for example, it is 14258 then star twice to blacklist the number that last called. (On another provider, just to show the variation, it is 1572 then 1.)
The downside of a single button would be that even with the existing systems it is all too easy to accidentally blacklist the wrong people. I know someone who was accidentally blacklisted by one of xyr elderly friends. It caused all sorts of interesting fallout until the two of them had the opportunity to speak in person.
One can imagine what a single button could do in the hands of small children. And, indeed, we have known for a couple of decades what anti-spam buttons can do in the hands of people who (say) decide to "junk" every mail message after reading it.
For Australia, see rather the Do Not Call Register, which is the better option as these blacklisting systems are usually restricted to a couple of handfuls of numbers.
