I've said this one before, but I'll repeat.
Everyone on my contacts list has a unique ring tone. I really don't care which
one it is, just that it is not the default "ring . . ring . . ring". If any call comes in
that is ring-ring-etc. I do not answer, which puts them into voice mail.
(I use the [your number] is not available . . . as the voice mail "greeting")
When I receive a voice mail, I listen to it and, if it is someone I want to talk to,
I add them to my contacts list (with any unique ring tone), and then call them
back. - - - If it is anyone I do not want to talk with, I block them, delete the phone
mail, and clear the call history.
When I started this process, I was receiving from one to as many as six bogus
calls per day. I am now down to one or two per month. - - - It takes time, but since
you never talk with anyone you do not want to, it is worth it. (IMO)