[Rjevski]

You connect to other carriers directly. Some of them will connect you to other carriers, and so on.

So for example if you’re a small operation you will just connect to one carrier like Twilio or Nexmo and it’s up to them to connect you to everyone else.

When you get bigger you may be able to set up “routes” (that’s how they’re called in the industry) to carriers directly.

So yes, carriers do see where the calls are coming from, however given that it’s commonplace to route your calls through a carrier that has no relationship to the originating number, there’s no easy way to tell whether a number is spoofed or not.

There is a solution which is to respond to complaints and stop interconnecting with a carrier that spews too much garbage, but that would go against the entire industry’s business model so nobody cares to do it.