In the particular case of Spain, student and teacher difficulties associated with the mathematical discipline have been evidenced in PISA and TEDS-M reports. As we consider that the teachers' difficulties are connected to the students' performance, we propose a multi-disciplinary approach to deliver specific didactic/mathematical knowledge to the trainee teachers. Such additional instruction shall be meaningfully connected to the real needs of the schools, so a service-learning approach is proposed here. In the present manuscript, the trainee teachers have co-designed educational escape rooms (in coordination with local schools) with the aim of mobilizing curricular knowledge. The goal of the educational escape rooms is to foster the mathematic-related competencies by establishing meaningful connections to other curricular disciplines (music-related knowledge, in the case of this study). This paper reports on the particular experience developed with a group of students (trainee teachers) while designing their educational escape rooms, focusing on the particular case of a specific student to evidence the formative potential of the procedure. The didactic suitability of the proposed escape room has been analyzed and professional development has also been discussed, showing the mobilization of relevant professional skills and fostering the related music and mathematical didactic competencies by shifting the teaching perspective from an algorithmic point of view to a more "reasoning and designing" strategy. This constitutes an evidence of the formative potential on the co-design of educational escape rooms, when designed in the frame of a service learning approach.