Most younger people (< 30 years old) already do this anyway and refuse to answer phone calls from numbers that are not already in their contact list. I know I do and I'm not young anymore.
Unfortunately that stops being an option when your parents become elderly and missing a emergency call from an unknown number has unpleasant repercussions you would like to avoid.
Set your voicemail message to tell them to leave a message or to send a text.