An unassigned block is a range of numbers that a regulator's published allocation table shows as belonging to no operator — free, protected or quarantined. No carrier holds it, so no subscriber can be reached on a number inside it.
What Spaw answers
block_status is unassigned and the reason is unassigned_block. The number is well-formed; it simply cannot be in service.
The gate on inferring it
A number's absence from a table is only evidence when the table claims to be complete. Five of the eight regulators' tables installed here do: the two North American allocation files list every assigned code, as do the French, Dutch and Mexican ones, so a geographic number missing from those can be reported unassigned. The UK's tables list free, protected and quarantined ranges explicitly and are never inferred from — an absent UK range answers null rather than a guess, and the Australian and US-address tables are read the same cautious way.
There is a second gate. A table that has not been synced inside its freshness window is not allowed to infer anything either, because a stale copy is exactly how a newly-allocated block gets reported as dead.
Where no table covers the region, block_status is null. Null here means "not evaluated", never "no".