The phone company does unfortunately allow spam callers

I'm honestly quite sick of this argument. The issue with the phone company isn't the spam. It's the anonymity. All solutions to the spam issue don't deal with unwanted calls at all.

Is gmail allowed to filter out spam? Or does doing so make them an email publisher no longer exempt from liability for email contents?

Really this is patchwork on what needs to be a major regulatory overhaul.

Things that need to be addressed: no advertising to kids, mob control, fake accounts/social manipulation, free speech, transparent and auditable algorithms (screw their IP). Its not an easy task to achieve all of these and other goals but it needs to get done before society literally goes into a civil war.