I've quit answering any unknown calls due to this out of control robocall spam. At first I felt tinges of anxiety thinking that I might miss an important call, but that passed very quickly. Now I figure that if it's really important the interested party will find another way to reach me (email, sms) and I feel zero anxiety. It's an interesting change in behavior. I'd almost be willing to use an app that restricted calls to my address book plus a whitelisted group of vetted companies.