HLR Lookup is a way to determine the home network of a mobile subscriber. HLR is also used to find what country the subscriber is roaming in right now. This technique is normally used together with SMS Notification and SMS Marketing to check what network you should send the messages to. It can also be used to check if a subscriber is reachable or if the user is absent (i.e. has turned off the phone). This can essentially lower your expenses by clearing your database from wrong or absent numbers and send messages only to live numbers.