This is incorrect. Easy access to SIP trunks is the problem, SIP trunks that don't enforce a connection between ANI, BTN and Caller ID. The telephone network

This is incorrect. Easy access to SIP trunks is the problem, SIP trunks that don't enforce a connection between ANI, BTN and Caller ID. The telephone network was never designed for non trusted parties to have access to the signaling system for the PSTN. Neither analog lines, nor TDM ones need much in the way of authentication, or encryption - because of that tie to a physical line, and a physical nexus. Very little if any spam/swat calls are coming from the analog side of the PSTN, a marginally larger but near zero amount comes from traditional TDM services, the bulk of it is shoddy SIP carriers.