Open source practices and tools have proven to be highly effective for overcoming the many problems of geographically distributed software development. We know relatively little, however, about the range of settings in which they work. In particular, can corporations use the open source development model effectively for software projects inside the corporate domain? Or are these tools and practices incompatible with development environments, management practices, and market-driven schedule and feature decisions typical of a commercial software house? We present a case study of open source software development methodology adopted by a significant commercial software project in the telecommunications domain. We extract a number of lessons learned from the experience, and identify open research questions.