When the STIR/SHAKEN standards were being defined, they were written as if from a single country perspective because it was intended to eventually be deployed abroad country-by-country.
The scope of the standards was designed to consider each country’s individual needs and their number ownership. The standards can be tweaked to fit regulatory requirements and governance, while ensuring that at the protocol level it will work cross-border.
Currently, cross-border calls will not be blocked by terminating carriers and will complete either without attestation or with a Level C. The only call blocking that may results is not STIR/SHAKEN related, but may be the result of third-party call apps or analytics.