NPA-NXX
The first six digits of a North American number: the area code plus the exchange, which together identify the block a number was assigned from.
In the North American Numbering Plan, NPA is the three-digit area code and NXX the next three digits. Numbers are allocated to carriers in blocks keyed on this six-digit prefix, which is why the prefix reveals the carrier the block was assigned to and the rate centre it belongs to.
This data is published, which makes geographic and assigned-carrier lookups possible without paying a per-query vendor. Our engine resolves against roughly two hundred thousand prefixes refreshed daily.
The limit is portability. Once a number moves to a different carrier, the block assignment still names the original owner. Knowing the current carrier requires portability data or a live network query.