This is a problem created and continued by phone companies. It wasn’t a huge problem before VoIP because the phone company networks didn’t allow everyone to forge sender info (SS7, not CallerID). When VoIP came along they didn’t put strict authentication in because it would have cost more, and that really opened things up.
Right now, you could have extra caution for 911 calls which originate from VoIP. Pass a law that in five years telco CEOs are charged as assisting murder the next time a SWATing happens and they’d magically discover that egress filtering is easy to implement once you’re not looking at the revenue generated by spammers.