A number inventory system is disclosed. The number inventory system manages the relationships between SIM cards, IMSI numbers, and MSISDN for a mobile telecommunications service provider. The number inventory system provides the software and hardware for receiving SIM card inventory data, including associated IMSI numbers, and efficiently stores the SIM card inventory data in a database. The number inventory system further provides for the opening of a range of available MSISDN. SIM cards, IMSI and MSISDN may be assigned to customer orders, and the number inventory system manages the associations between IMSI numbers and MSISDN and monitors the status of SIM cards, IMSI numbers and MSISDN. The number inventory system may also manage an MSISDN classification system for identifying MSISDN numbers having various desirable sequence characteristics.