Because of how Telecom works, robocallers can put whatever originating number they want in the SIP header. This will eventually come to your phone, usually as a

Because of how Telecom works, robocallers can put whatever originating number they want in the SIP header. This will eventually come to your phone, usually as a number in the same NPA-NXX (area code, first 3) to trick you to pick it up. Carriers that originate a call can know if their customer doing so owns the number, but all the other carriers along the many hops it may take，have no way of verifying that one of the previous carriers owns that number. The solution is a combination of the Stir/Shaken implementation and the proof of ownership for telephone numbers that the FCC is starting to ask carriers to implement. Anyways, because people can fake the orig number, and SS7 is massive, it's nearly impossible to track down violators like this, given a number.