Hutchison Telecom Hong Kong had a problem. The telecoms Regulator, OFTA, wanted to take away some of its spectrum and use it to add yet another competitor into this already highly-competed market. Hutchison perceived that the proposed action would not only be unfavourable for Hutchison, but also for the consumer. But this view hadnít been accepted by the Regulator, when expressed in the form of traditional regulatory arguments. This case study describes how Hutchison commissioned and used a System Dynamics simulator of the Hong Kong wireless markets (2G and 3G, voice and data) to rigorously and transparently quantify the situation. The simulator used 1) interviews with many experts and stakeholders, including the regulator, 2) confidential company data, appropriately protected, 3) judicious calibration against 2G history and 3G plans, 4) optimization of 3G competitorís pricing and investment strategies to ďgame outĒ future market evolution, under different regulatory decisions. Sensitivity testing showed that the remaining uncertainties did not alter the fundamental results: The regulatorís proposed action would not benefit the public. After due consideration, OFTA dropped its plans and will not bring in more 3G carriers. Both Hutchison and OFTA have done well for their respective stakeholders.