The ubiquitous presence of cell phones in emerging economies has brought about a wide range of cell phone-based services for low-income groups. Often, the success of such technologies depends highly on their adaptation to the needs and habits of each social group. In an attempt to understand how cell phones are being used by citizens in an emerging economy, we present a novel methodology to analyze large-scale relationships between specific socioeconomic factors and the ways people use cell phones. Our approach combines large-scale datasets of cell phone records with countrywide census data to reveal findings at a national level. We evaluate the proposed methodology in an emerging economy in Latin America and show relevant correlations between socioeconomic levels and social network structure or mobility patterns, among others. Finally, we provide an analytical model to formalize the relationship between cell phone use and demographic or socioeconomic variables.