Public health promotion technology should be accessible to the general public at which it is aimed. This paper explores the potential for use of an unaugmented commodity technologya??the mobile phonea??as a health promotion tool. We describe a prototype application that tracks the daily exercise activities of people carrying phones, using fluctuation in signal strength to estimate a usera??s movement. In a short-term study of the prototype that shared activity information amongst groups of friends, we found that awareness encouraged reflection on, and increased motivation for, daily activity. We describe some of the details of the pilot study, and conclude with our intended plans to develop the system further in order to carry out a longer-term clinical trial.