The Ministry of Information and Communication introduced, in 2004, a mobile number portability system in the mobile communication industry in an effort to make consumers to select mobile service provider of his/her own choice without changing personal cellular phone numbers. Despite the purpose of the number portability, SK Telecom had to execute its defensive marketing for the first 12 months, while its competitors actively attract SK Telecom’s subscribers. Twelve months later when SK Telecom is allowed to draw customers from its competitors, there is a controversy inside SK Telecom whether or not to take an offensive marketing strategy to raise its market share to the level it enjoyed before. This concerns stems from the fact that the mobile telecommunication industry is heavily regulated by the government, mainly due to limited frequency available to the operators. Regulation includes the issues of access charge, pricing, and business licensing. So for a mobile service provider, it’s important to consider the policy and regulations by the regulating bodies on all decision-making process. Particularly for a market leader which is being monitored by regulatory institutes, government relationship is of critical importance in marketing.This case aims to discuss how to build and carry out the marketing strategies considering government regulations as well as competitors’ strategic moves. This case specifically deals with the situation under which SK Telecom had to drop its market share down under 50% in 2000 when the regulatory body allowed SK Telecom to take over Shinsegi Telecom with some specific conditions.