We study the effect of a randomized one-on-one remote phone tutoring program implemented between 2021 and 2023. The intervention reached almost seven thousand students in seven Latin American countries: Argentina, Brazil, El Salvador, Guatemala, Mexico, Paraguay and Peru. The program targeted students with low initial learning levels and focused on foundational numeracy skills using a differentiated instruction approach. We find that assignment to tutoring increased student test scores by 0.2 SD. Tutoring benefited all students, with no differential effects by gender, age, socioeconomic status, or baseline scores. Students who initially reported having difficulty with concentration or memory experienced larger average effects. Finally, we find that students with lower initial performance exhibited larger improvements in more basic mathematical operations, whereas those with better performance at baseline saw larger gains in more complex operations. This underscores the importance of offering differentiated instruction based on students‘ initial performance.