I agree with droopy that spoofing a number must be declared illegal.
It is illegal, if used for illegality; just unpreventable. Cybercrime has proved legislation is not a deterrent. Moreover, a law in one country may not be law in another.
Unpreventable doesn't mean that it's not worth a go. Many laws are in place as a deterrent and while they may not deter everyone they will deter some. Another reason for a law is that when someone seriously crosses the line (i.e. prosecution is in the public interest) the authorities have something to prosecute.
I think the real problem here is the ability to pick up a phone without any form of identification> Identification could be as simple as a driving licence or using a credit card. While some criminals will of course still get around this, it will catch a lot and will deter a lot more.
trewmte is right in his analysis.
Voice biometrics aka VoicePrint will solve the problem theoretically - but not in real live. As long as OTT applications out of messengers (to chat is the new black for years) dominate the field, the problem is not solvable.
But voice-based services are on decline as they require a dual-availability and do not offer a delay time to reflect the content or bundle/sort out immediate emotions, to text-based chat is already today a common tool of extortion, duress and threats.