Examples of mobile phones being used with teachers to provide continuing professional development (CPD) in emerging economies at scale are largely absent from the research literature. We outline English in Action's (EIA) model for providing 80,000 teachers with CPD to improve their communicative language teaching in Bangladesh over nine years. EIA's CPD program is delivered face to face and supported through open distance learning (ODL). This innovative model of teacher CPD is supported through peer learning and self-study using a variety of print, audio and video resources. Drawing on the success of EIA's pilot studies, where internal and external evaluation reported significant improvement in teachers' and students' English-language competence after one year, the current phase is using low-cost mobile phones, or the "trainer in your pocket" to deliver CPD to 12,500 teachers through 2015. We believe EIA's teacher CDP model is best suited to assist the project in achieving one of its primary goals: to increase the English-language proficiency of 12 million students, allowing them to access greater social and economic opportunities in the future. We argue EIA's use of mobile phones for the provision of teacher CPD -- at scale -- is timely and replicable in both developed and developing contexts. (Contains 3 figures and 2 notes.)