This article investigates working-memory (WM) failure in phone-based interaction (PBI). We used a computational model of phone-based interaction (PBI USER) to generate predictions about the impact of three factors on WM failure:PBI features (i.e. menu structure), individual differences (i.e., WM capacity), and task characteristics (i.e., number of tasks). Our computational model stipulates that both the storage <italic>and</italic> the processing of information contribute to WM failure. In practical terms the model and the empirical results indicate that, contrary to guidelines for the design of phone-based interfaces, deep menu hierarchies (no more than three options per menu) do not reduce WM error rates in PBI. At a more theoretical level, the study shows that the use of a computational model in HCI research provides a systematic approach for explaining complex empirical results.