This chapter describes the design and implementation of a softswitch for UMTS developed by Computer and Communication Research Laboratory (CCL) of Industrial Technology Research Institute (ITRI). This softswitch can be utilized as call agent (media gateway controller) for the third generation mobile all-IP network such as UMTS. The CCL Softswitch follows the reference architecture proposed by International Softswitch Consortium. In our approach, the Intelligent Network (IN) call model is implemented to interwork with existing IN devices. We design protocol adapter modules and service provider interface to ensure that multiple VoIP protocols can be supported without modifying the core of the softswitch. Furthermore, the message flows of call setup, call transfer and inter-softswitch call are described to show the feasibility of the softswitch.