The problem with phone is bad VoIP verification. For a while, I tried to call back unknown numbers who had called me. A lot of them were just not assigned. But a less reputable telco company can still allow their customers to use it as the "from" field for a VoIP call. Otherwise, the solution would be very straightforward: Allow regular people to sue a given phone number for spam/fraud and then the government (who has that info anyway) will disclose the phone number's owner to the judge. But that only works if the "from" phone number cannot be as easily falsified.