O2 is spending £10m revamping it's mobile registration database system (know as an HLR or Home Location Register) as it has suffered 2 major outages in as many months. The HLR stores the customer IMSI (SIM number) and their real telephone number as well as location and other information. When a call comes in, the HLR is queried to find out how do deliver the call the mobile. If the HLR fails, then mobile calls will not work (as mobiles cant register on to the network) and calls to mobiles will fail too. The current HLR (or cluster of HLRs) are made by Ericsson and O2 is buying new systems.