However there are other tiers of attestation that are less strong, and because telcos also do a screaming business in bulk transport of other people's calls, these calls still get connected. So for example user C in say, Telenor Pakistan calls user A on AT&T, but the call is carried across the world by some transit carrier, like Lumen or BICS. This happens all the time. Then all that ATT see is that BICS attest that they trust Telenor, but have no control over the source number C.
It gets real murky real fast. On top of all this there are yet more complex cases, like American Express buying a block of phone numbers from one phone company but actually being connected to the global phone network by another. Or wanting to have those domestic numbers route offshore but still appear as US numbers when they call you stateside.
Its a mess, and SS helps as best as it can, but I think the real solution requires a change in how telcos get paid, and route one another's traffic for money, and that is not changing anytime soon.