Formal methods have been used mostly in safety-critical projects although some have industrial-strength implementations that can be used effectively in commercial settings. This paper describes a case study conducted in collaboration with Nortel to demonstrate the feasibility of applying formal methods to telecommunications systems. A lightweight formal method, SDL, was chosen by our qualitative evaluation to model a multimedia-messaging system. The model allowed us to locate errors in the software requirements document and to derive test suites, shadowing the existing development process and keeping track of a variety of productivity data. The success of our case study was in finding a suitable project where we were able to integrate a well-chosen formal method into the existing development process.