Its not simple at all. The vast majority of spam calls I get are from legitimate institutions that I'm sure would pass whatever a STIR is.

Its great that your insurance providers can call you, what's to stop an insurance provider who is not yours from calling you?

A legitimate caller for one person can often be a spam caller for 100 other people. That's why there exists no way to possibly differentiate one spam caller from another.

If my insurance provider calls me from an unknown number, they leave a voicemail and I call them back later, or send them an email.