Pakistan's mobile phone market is one of the world's fastest growing markets with a subscriber base of 137 million users. Competition in the country's telecommunication industry is dominated by four players and customer demand is high. In addition, Pakistan is the first country in South Asia to have implemented Mobile Number Portability (MNP) in March 2007. MNP is a facility that allows mobile subscribers to switch between service providers without changing their existing phone numbers. Mobile phone service provider's selection may be influenced by various factors. Therefore, this research was undertaken to explore factors affecting MNP and to determine which factors were most influential in the selection of mobile phone service providers in a developing economy like Pakistan. Moreover, the study also probed into differences in customer perceptions between prepaid and post-paid customers. This exploratory study is based on primary data collected from 300 customers using services of different cellular companies who had experienced MNP facility. Factor analysis was carried out on obtained data and reliability of the resultant scale was verified to achieve the objectives of the study. The outcome of this research provided a concise framework of the various dimensions of customer choice. Contrary to previous researches, the result of this study indicated that infrastructural services, customer relationships, call quality and promotional packages were the most important factors affecting MNP. However, price of services was found less important factor in selection of telecommunication service providers. This study also showed that pre-paid and post-paid customers can be significantly different for many value-added services and promotional tools.