"Oh, you can't go back to knowing who is allowed to send what number. Things are too mixed up, and it'd break many services. Including many uses of 9-1-1. It's

"Oh, you can't go back to knowing who is allowed to send what number. Things are too mixed up, and it'd break many services. Including many uses of 9-1-1. It's far more difficult than, for instance, preventing spoofed IP source addresses." That's interesting. Why is that?