Unfortunately sometimes important callers use anonymous numbers. I recently had to deal with this in the public healthcare system where I live (not in the US).

A certain person who needed to give me an appointment kept calling at unpredictable afternoon and evening times from an anonymous number, ignoring the voicemails I left with my availability info, and not proposing specific times in her voicemails to me.

I did get the appointment in the end, but only by happening to notice one of her calls in real time - still from an anonymous number - and answering.

I'm told similar important uses of anonymous numbers aren't too rare in this public healthcare system.