Mobile phone data has become a valuable resource in mobility studies, providing a rich dataset for analyzing human movement. This study presents a comprehensive review of mobile phone data sources and their applications in transportation studies. It explores the strengths and limitations of data types such as call detail records (CDR), cellular signaling data (CSD), and synthetic CDR. The study discusses methodologies for estimating travel demand, generating origin-destination (OD) matrices, and conducting mode-specific OD estimation. While CDR data has proven reliable for trip purpose estimation, challenges remain in accurately predicting travel modes. The study concludes with recommendations for enhancing synthetic CDR data and expanding its use in mobility studies.