[viraptor]

It's not that easy. You can get a call from X with a prefix from B and it's just how routing sometimes works. In the same way an internet connection from Germany to the UK may actually arrive from an interconnect in Amsterdam. But it doesn't matter. What matters is the responsibility of each party to point out who sent the call to them.

(And that's not even mentioning the issue of how you'd map providers to prefixes. It's somewhere between non-trivial and impossible in practice.)

[TheSoftwareGuy]

It should really be more like how the internet works with IP addresses.

Its easy to send an IP packet with a source address that is whatever you want, but if you do that the receiver is going to reply to someone else and you won't be able to establish two way communication.

[musicale]

Ideally the ISP does basic ingress policing and doesn't accept packets from source addresses outside its range.

Yes, I know most ISPs don't do this. But they could if they wanted to.