This longitudinal study explores how the language proficiency of eight (N = 8) US-affiliated university students developed relative to their smartphone usage during study abroad (SA) in Paris. Phone usage was tracked daily with Space, proficiency weekly using NCSSFL-ACTFL Can-Do statements, and language use monthly via a Language Engagement Questionnaire. Adopting a Complex Dynamic Systems Theory (CDST) framework and following a microgenetic approach to case studies, the trajectories of each participant's phone consumption and Can-Do statements were established, allowing to visualize floors, ceilings, and probes in proficiency development. Spearman's correlations served to determine whether and how phone usage related to fluctuations in proficiency, and Kendall's tau-b its link with language used. Findings revealed that these relationships varied widely across participants, types of phone usage, and phone applications, with students displaying positive, mixed, or negative relationships, thus suggesting that there is no one-size-fits-all model to proficiency/smartphone management, but idiosyncratic complex cases.