Telecollaboration, defined as a structured online exchange between classrooms in different cultural and linguistic contexts (Belz, 2003; O'Dowd & Ware, 2009), is now recognized for its potential in training future teachers (Emir & Yangın-Ekşi, 2024; Coşkun, 2023). However, our study explores the impact of designing and implementing telecollaborative projects on the professional development of in-service teachers, an area that has received little attention in research so far (Nissen & Kurek, 2020; O'Dowd & Dooly, 2021). Our research is based on a longitudinal analysis of competency development among teachers involved in eight telecollaboration projects. A literature review was first conducted to identify the competencies most likely to evolve and their various dimensions. We selected three key competencies: techno-pedagogical, pedagogical, and intercultural. Next, we identified relevant variables based on established frameworks. Techno-pedagogical competence was assessed through the European Framework for the Digital Competence of Educators (DigCompEdu) (Redecker, 2017). For intercultural competence, we drew mainly on Byram (1997), who stresses the importance of fostering a reflective stance on teaching practices. Teaching competence was examined through the lens of Bandura's (1997) concept of self-efficacy, which is shaped by teachers‘ experiences and learners’ feedback. We employed a mixed-methods longitudinal approach, combining surveys and semi-structured interviews conducted before, during, and after each telecollaboration project. This methodology enabled us to monitor the development of teachers' skills and the strategies they adopted in response to various challenges. Preliminary results suggest that participation in telecollaboration improves teachers' mastery of digital tools, and their confidence in their teaching practices.