Hard filtering of calls

I find that, over time, my cell phone number has propagated out to spammers/scammers whom want to call me up to sell me something. The US national do-not-call registry hasn’t helped. The complaints I’ve filed haven’t helped.
So I filter. My filtering algo looks like this:

if (number_is_known_person_or_org(phone_number)) {
else if (number_is_unknown(phone_number)) {
function filter_stage_2(phone_number) {
// I ignore 80% of numbers I don't know, let them go to
// voicemail.  Voicemail is an excellent filter, if you
// don't leave a VM, then its obviously not important enough
// for me to pay attention.
if (roll_the_dice(bias_to_ignore=0.8)=="answer") {
function answer_phone(phone_number) {
// You have 15 seconds to state your name, your
// affiliation, and what this call is about.  Failure
// to do this in 15 seconds, and I will hang up.
// This means that all those calls for "the owner" or
// other such BS, if they get answered, are hung up on
if (the_other_party_explains_why_I_should_spend_my_precious_time()) {

The 80% figure used to be 30%, then 50%. Its been 80% for a while. In short order it is going to 100%. If its important enough to call, you can leave a message. I will get it, and if its important to me, I will respond.
Sadly, android doesn’t quite have a “block calls from this number” in its basic phone app (gee … seems pretty obvious that most people want this …).
So far this year … this year, both of the calls that I answered came through from the same person, with the same script, and I patiently explained that they had to tell me who they are, what they were calling about. They refused, saying only “this call is recorded”, and giving me the name of an entity that shouldn’t be calling me.
It used to be “charities”, then it was scam artists, then it was others.
What is going on is convincing me that I do not need a public cell phone number, and that I should give out my voicemail to all whom request my number.
About 3 years ago, we changed our home phone over to VOIP, and I turned off the ringer. This was a few months before the last election cycle, and suddenly we weren’t wasting time as a family fielding calls from places that shouldn’t be calling us. We don’t miss the home phone. It is exceptionally rare that we get a meaningful voicemail on it. Most are marketeers/scam artists.
The filter is pretty good, but I am thinking of simply ignoring any call I do not know the number for. If they leave a VM, I will get it, moments after the call itself, and I can decide whether or not to call them back.