This document summarizes a project using model-based testing (MBT) to test the charging subsystem (CS) of a mobile operator. Key points: - An MBT approach was used to generate automated test cases from a state machine model of the CS to test it more quickly and thoroughly than manual testing. - Over 12,000 test cases covering all transition triples were generated and executed in under 6 hours, finding 15 new errors. This significantly improved test coverage compared to manual testing. - The mobile operator was convinced of the benefits of MBT for reducing testing time and costs while improving quality compared to their previous approach. They plan to use MBT for testing in the future.