It's at the carrier level. I believe that at least some part of it is noticing that the calling number is a number that doesn't exist. For example AT&T might be

It's at the carrier level. I believe that at least some part of it is noticing that the calling number is a number that doesn't exist. For example AT&T might be given phone numbers 555-0000 through 555-1000, and they know they've only actually given out 555-0000 through 555-0782, but the caller ID claims the number is 555-0999. They can tell it's not a legitimate call because that number couldn't be making an outgoing call because it's not assigned. I don't think that's all of it by any means, but there are a number of ways that the carriers can tell that some percentage of the calls are bogus without having to ask the receiver if they want to receive it.