The real solution is to have an authenticated caller ID system. The regulatory organisation responsible for the country's phone numbers holds a root certificat

The real solution is to have an authenticated caller ID system. The regulatory organisation responsible for the country's phone numbers holds a root certificate, and as number ranges are allocated they issue a certificate to the new owner. This cert can in turn be used to sign more certs as this new owner is in turn allocating numbers to their customers. When a call is placed the originator uses its own certificate (issued to them by their phone provider) to sign the call request and this can be verified by any carrier in the path of the call. Any unsigned caller ID gets flagged or the call is outright dropped.