This study examines the effect of management accounting information practices on managerial decision-making effectiveness in selected telecommunications firms in Nigeria. A quantitative cross-sectional survey design was adopted. Primary data were collected through structured questionnaires administered to managerial and supervisory staff. Using Yamane’s (1967) formula, a sample size of 150 respondents was selected through stratified random sampling from major telecommunications firms, including MTN Nigeria, Airtel Nigeria, Globacom, and 9mobile. Data were analysed using descriptive statistics, Pearson correlation, and multiple regression techniques in SPSS Version 23. The findings reveal that information collection and information communication exert positive and statistically significant effects on decision-making effectiveness. Conversely, information processing and report quality demonstrate weak or statistically insignificant effects. The results support Decision-Usefulness, Contingency, and Bounded Rationality perspectives by demonstrating that accounting information enhances decision outcomes when it is relevant, accessible, and clearly communicated rather than analytically complex. The study contributes industry-specific evidence from a developing economy and provides practical insights for strengthening management accounting systems in technology-driven service environments.