HLR lookup

A live query to a carrier's Home Location Register that reveals whether a mobile number is active and which network it belongs to.

The HLR (Home Location Register) is the database where a mobile network keeps its subscribers. An HLR lookup asks it, in real time, whether a number is live, which operator serves it, and whether it has been ported.

Platforms and SMS senders use HLR data to skip dead numbers and to detect number types before sending. It is one of the mechanisms behind "this number is not accepted" errors.