For those who have smart phones, just don’t answer any phone calls from numbers not in your contact list. If it’s a legitimate call, he/she can leave a voicemail. The only exception I make is if I’m expecting an important call. I have an iPhone 8. I have toggled a switch in the Settings that will silence phone calls from numbers not in my contact list. The number is visible and a voicemail is available for the caller. Of the past approximately 100 silenced phone calls, only one was from a friend who was not in my contact list. She left a voicemail.