Ethiopian Telecommunication Corporation has lost around 5 hundred million birr in 2006 because of the loss of billing data. This thesis discusses the technical, operational and managerial aspects of the billing system for Ethiopian Telecommunication corporation. The research methodology used during the study was qualitative case study. The data was collected using questionnaires, interview and observation. Based on the data collected the findings indicate that there is a security understanding gap on technical, operational and managerial staffs. Hence, adopting the ISO 17799 security framework is recommended to come up the problems.