Acceptance and use of an information system always remained an important concern for practitioners and theorist. Developing countries like Pakistan are far behind in the acceptance of information system. This study explores the factor affecting user behavior toward use of ERP system. Performance expectancy, effort expectancy, social influence, facilitating condition, project communication, training, top management support and self efficacy were found frequently used factors to investigate the ERP usage behavior. This study also investigates the impact of behavior toward ERP system usage on the employees'' performance. The findings indicate that the proposed model over all explains 71% variation in the behavior. While Performance Expectancy, Self Efficacy, Training, and Top Management Support were significant factors while explaining the user behavior toward use of ERP system. Mostly respondents agreed that they like the idea of using ERP system and they want to become professional in ERP system. This research will help the telecom sector to understand the factors responsible for the employee''s acceptance toward use of ERP system in developing countries.