A cryptographic solution is absolutely necessary. Most reputable telcos already restrict spoofing or require tons of paperwork to prove you own the number befor

A cryptographic solution is absolutely necessary. Most reputable telcos already restrict spoofing or require tons of paperwork to prove you own the number before allowing you to use it as caller ID (like Twilio for example). The issue is that the PSTN is essentially a huge, worldwide message queue to which pretty much any telco can connect around the world, including shady ones - even if US law actually does fight spoofing, how do you prevent telcos from other countries from continuing the abuse? Cryptography is needed - when a carrier leases you a number, they give you a certificate with which you can sign other carrier’s certificates if you want to let them use that number as caller ID. Every carrier on the call chain should verify call’s signatures against that and discard any calls with missing or invalid signatures. That will stop malicious spoofing while allowing its legitimate use, just like email where you can use SPF and DKIM to nominate any email provider to be able to send on your domain’s behalf.