A live carrier check, often called an HLR lookup after the home location register in which a mobile network keeps its subscribers, is a query sent into the carrier networks at the moment of the lookup to learn whether a number is in service now, which network serves it after any port, and whether the handset is roaming. Numbering plans and regulators' allocation tables cannot answer those three questions: they record how a number was allocated, not what is happening to it today.
What Spaw answers
The live carrier-network check (hlr: true) is not enabled on the service: hlr_checked answers false and nothing extra is charged.
The API accepts hlr: true on single, batch and bulk lookups, and monitored lists take live_check. The four fields the check would fill are reachable, ported, current_carrier and roaming; each is null until a check runs, so every answer today carries them null. A lookup made with hlr: true answers exactly as one made without it.
Everything else in the answer is read from local data. carrier names the operator the number was allocated to, from the regulator's block holder where a table covers it and from the numbering plan's prefix metadata otherwise, and a number ported to another network keeps the block it was born in (see NPA/NXX). Only a query into the carrier networks, an HLR or number-portability lookup, can say who serves it today, which is why the check's answer has fields of its own and is never merged into carrier.
One rule of the SMS policy depends on the check. require_reachable refuses a number unless a live check reached the handset: blocked_by: "unreachable" when the network could not reach it, "unconfirmed" when no check answered. While the check is not enabled, every lookup answers unconfirmed under that rule, so leave it off.
When a check runs, it is the one call that leaves Spaw's servers: it sends that number, and only that number, to a carrier-network partner, and the network's answer is held for a day under a hash of the number. A number in a block nobody holds, or in a range reserved for fiction, is never checked.
What it does not mean
A reachable number is not a verified owner. The check reports what the network knows about the subscription, not who is typing the number into a form, and it is not SIM-swap detection.
It names nobody. There is no caller name, no reverse lookup and no subscriber detail in any answer, with or without the check.
Without it, valid: true means a number is assignable, not that it is in service. Which regulator tables Spaw reads for each country, and what they can settle, is under phone facts by country.