HLR (Home Location Register) is a database that contains various information about all of the mobile subscribers of a mobile network such as the mobile numbers, services, whether the numbers have been ported to another network and similar information. Home Location Register lookup (HLR Lookup) is a service that dynamically contacts a central database (on the operator’s side) that contains details of each mobile phone subscriber authorized to use the GSM core network.
Cell phones and their corresponding SIMs are mostly on the move. The HLR is used to identify their last known location.
The HLR also facilitates Short message services (SMS) by scanning through the HLR to identify the mobile switching centre (MSC) currently used by the receiving party. If the recipient is reported as not available by the receiving MSC, a message waiting flag is sent on the HLR. The message is then delivered once the recipient is located on any MSC.