A Home Location Register (HLR) is the central database containing details of each mobile phone subscriber authorized to use the GSM core network. Each mobile network has one HLR, although there may be many HLR servers creating a single virtual HLR. A HLR stores details of every SIM card issued by the mobile phone operator. Each SIM has a unique identifier called an IMSI which is one of the primary keys to each HLR record.
A HLR stores the following data about a mobile subscriber:
Current servicing MSC status of the subscriber
Temporary handset error code if applicable
Status – valid/invalid
If the subscriber is available to receive SMS
If there is an error code preventing SMS delivery
If the number has been ported to another mobile network.