Mobile health (mHealth) applications have proliferated across the  globe with much enthusiasm, although few have reached scale and shown public health impact. In this study, they explored how different contextual factors influenced the implementation, effectiveness and potential for scale-up of welTel, an easy-to-use and evidence-based mHealth intervention. WelTel uses two-way SMS communication to improve patient adherence to medication and engagement in care, and has been developed and tested in Canada and Kenya.