The switch services software of System 75 provides the basis for an extensible office communication system, supporting a wide variety of voice and data‐switching services. This paper presents the software architecture of the System 75 switch services. The concepts of user, group, and process‐per‐call form its foundation. We introduce the architecture by stepping through a simple station‐to‐station phone call, and proceed to the derivation of a call model based on the topology of a call. This call model is realized as a layered set of cooperating processes that execute under the Oryx/Pecos Operating System on the System 75 switch processor. The software layers and processes are discussed and a call walk‐through is used to illustrate the process interactions.