HLR lookup
A live query against the mobile network's Home Location Register to see whether a handset is currently reachable and which network serves it.
The Home Location Register is the subscriber database inside a mobile operator's core network. Querying it returns whether the number is active, which network currently serves it after any porting, and whether the handset is roaming or switched off.
This is the only check that reflects the live state of a mobile number rather than an allocation record. It is also the reason it costs more: the query leaves our infrastructure and is billed by an upstream provider per lookup.
Typical use is deciding whether to spend money on an SMS. Validating format first at one credit, then running HLR only on the numbers that survived, is much cheaper than querying everything.