It'd work well for both ends of the spectrum (spam callers & friends) but think would kinda fall down in the middle though — kids' school, doctor's office, plum

It'd work well for both ends of the spectrum (spam callers & friends) but think would kinda fall down in the middle though — kids' school, doctor's office, plumber coming to service your boiler. Where you are expecting a call and it's definitely not spam, but they're not going to be ringing from a pre-approved whitelist. Don't think they'd want to be putting $1 per call on the line and hoping for people to remember to click the right button afterwards