Usability is a central issue for mobile phone design and service because users need to access various functionalities via limited user interfaces (UI) often while they are on the road. In this paper, we propose simplicity and interactivity as the key determinants of mobile phone usability and assess their significance in an empirical setting. Furthermore, we examine the effects of mobile phone usability on user satisfaction, trust, and brand loyalty and provide a holistic view of the causal relationships between the proposed UI features and important organizational variables for building and maintaining long-term customer relationships. The study was conducted using survey questionnaire data collected from 310 mobile phone users in South Korea. The findings of the study confirm that simplicity and interactivity are two significant determinants of mobile phone usability and that interface simplicity is an important precondition for positive interactivity and usability experience. Our findings also show that usability is a distal determinant of brand loyalty, exerting its influence indirectly through the mediators of satisfaction and trust. We discuss the implications of the study findings for usability research and UI design.