HLR Lookup Plus is simple application to getting info about Home Location Register (HLR)

The HLR (home location register) is a database mobile network operators maintain which contains each customer's international mobile subscriber identity (IMSI).

An IMSI uniquely identifies each customer of the mobile network and allows different network operators to send messages and calls to subscribers on other mobile networks.

Each subscriber's SIM card contains their IMSI which does not change. Because a network operator always references the IMSI of the SIM card then the customer of the network can then change their mobile number without changing their SIM card.