Over the last few years the FCC rules on carriers were relaxed so that they can discriminate against calls originating from domestic networks. Previously they could not block calls even if it was obvious it was a scam call. However, it's not always obvious which robo calls are legitimate and which aren't.

The bigger issue though is not about domestic origin calls, it's about foreign originated calls that the FCC has no power over. The domestic carriers had to forward these calls. There is traction gaining to require that calls that originate overseas verify their identity to US based networks. Domestic carriers can reject calls that don't verify (spoofed numbers that originate offshore).

That still may not stop all the spam calls but it will slow it down. The problem here is really that telephony became extremely cheap and accessible to the whole world.