I know on iOS you can just block any calls from contacts that are not in your list. It's pretty easy. My phone just won't ring. They'll be sent to voice mail instead. In my voice mail greeting, it says that I won't listen to the voice mail, and if they're trying to get in touch with me, hang up and text me.
Yeah, that is great that iOS did that! They released that last summer I think.
There are times that you do want anyone to call you, or you don't even want unknown callers to leave a voicemail.
My friend's mother would freak out with every little thing always, including the voicemails from robotic voices saying their SSN will be invalid unless they call back at some weird number.
So my friend got the app on her mother's phone and set it to the strictest mode, and her mom hasn't freaked out anymore. This only works because she uses her phone for talking with family members and a few doctor offices.
