The phrase "doing a service" can be replaced by several different synonyms depending on the context. Saying someone is "helping out" implies that they are providing assistance to someone in need. "Providing a favor" indicates that someone is doing something kind for someone else. "Performing a duty" suggests that someone is fulfilling a responsibility. Similarly, "conducting a task" could indicate that someone is completing a specific job. Using the term "rendering assistance" emphasizes that someone is being helpful. Finally, "providing aid" is another synonym that conveys the idea of helping someone in need. Overall, there are many different ways to express the idea of "doing a service".