I do not answer any phone calls from outside of my contact list stored on phone. If call is important they will leave me a message on voice mail.
I have a ringtone for my contact list and use the default one for everything else.
It is a pain to set the ringtone for every number on my contact list, but sometimes I’m somewhere where I have to wait and have time to kill and do it then.
Now I can tell by the ringtone if it’s something I want to answer or ignore.