At this point, I'm starting to think voice calls are a lost cause - most everybody I know doesn't even bother answering any more.
The one feature that would make my phone usable again is to have a simple option to only ring for numbers that are in my contacts - otherwise go straight to voicemail. I really, really want that. Why shouldn't I be able to decide who gets to interrupt me based on if I know them or not?
I'm under the impression that it's also possible to replace the dialer/call app with one that can detect "probable spam", and at least one such replacement can likely do custom ringtones or disable ringing for such spam.
https://support.apple.com/en-us/HT207099
> With iOS 13 and later, you can turn on Silence Unknown Callers to avoid getting calls from people you don’t know. This blocks phone numbers that you've never been in contact with and don’t have saved in your contacts list. If you’ve previously texted with someone using their phone number or if a person has shared their phone number with you in an email, a phone call from that number will go through.