# HLR - Home Location Register The [[HLR - Home Location Register|HLR]] is a central database that contains details of each mobile phone subscriber that is authorized to use the [[GSM - Global System for Mobile communication|GSM]] core network. There can be several logical, and physical, [[HLR - Home Location Register|HLR]]s per [[PLMN - Public Land Mobile Network]], though one [[IMSI - International Mobile Subscriber Identity|IMSI]]/[[MSISDN - Mobile Station International Subscriber Directory Number|MSISDN]] pair can be associated with only one logical [[HLR - Home Location Register|HLR]] (which can span several physical nodes) at a time. The [[HLR - Home Location Register|HLR]]s store details of every [[SIM]] issued by the mobile phone operator. Each [[SIM]] has a unique identifier called an [[IMSI - International Mobile Subscriber Identity|IMSI]] which is the primary key to each [[HLR - Home Location Register|HLR]] record. Another important item of data associated with the [[SIM]] are the [[MSISDN - Mobile Station International Subscriber Directory Number|MSISDN]]s, which are the telephone numbers used by mobile phones to make and receive calls. The primary [[MSISDN - Mobile Station International Subscriber Directory Number|MSISDN]] is the number used for making and receiving voice calls and [[SMS]], but it is possible for a [[SIM]] to have other secondary [[MSISDN - Mobile Station International Subscriber Directory Number|MSISDN]]s associated with it for fax and data calls. Each [[MSISDN - Mobile Station International Subscriber Directory Number|MSISDN]] is also a unique key to the [[HLR - Home Location Register|HLR]] record. The [[HLR - Home Location Register|HLR]] data is stored for as long as a subscriber remains with the mobile phone operator.