Poor public service provision and government accountability is commonplace in low-income countries. Although mobile phone–based platforms have emerged to allow constituents to report service deficiencies to government officials, they have been plagued by low citizen participation. We question whether low participation may root in low political efficacy to politically participate. In the context of a text message–reporting platform in Uganda, we investigate the impact of adding efficacy-boosting language to mobilization texts—(a) citizen name personalization and (b) politician encouragement— on citizens’ willingness to report service deficiencies to politicians via text messages. Both treatments, designed to increase internal and external efficacy, respectively, have a large, positive effect on participation. The results are driven by traditionally less internally efficacious constituents (females) and less externally efficacious constituents (those represented by opposition party members), respectively.