Improving “ last-mile” public service delivery is a recurring challenge in developing countries. Could the widespread adoption of mobile phones provide a scalable, cost-effective means for improvement? We use a large-scale experiment to evaluate the impact of phone-based monitoring on a program that transferred nearly a billion dollars to 5.7 million Indian farmers. In randomly selected jurisdictions, officials were informed that program implementation would be measured via calls with beneficiaries. This led to a 7.8 percent reduction in the number of farmers who did not receive their transfers. The program was highly cost-effective, costing 3.6 cents for each additional dollar delivered. (JEL O13, O33, Q12, Q18)