Which is also why it is simply an "initiator settable field". When "the phone company" brought out the service (for a monthly fee) there was only one "phone co

Which is also why it is simply an "initiator settable field". When "the phone company" brought out the service (for a monthly fee) there was only one "phone company" and so they could be assured that they themselves were setting the value to the correct source. Now that the phone network looks more like the internet (many different companies all exchanging "calls" with each other) that decision, way back then, has the unintended side effect of allowing the robocall spammers to set whatever set of ten digits they like on their outgoing calls.