This article analyzes the conflicts between institutions and actors responsible for the reception and the treatment of emergency calls in Paris, especially for medical emergencies, and the technical apparatus that makes possible these communications in urban space. Initially related to security and policing during the interwar period, the alert of emergency services became a medical issue in the 1950s. The management of alerts in the medical domain became thus an argument for the improvement of the efficiency of rescue services. The development of telephone equipment and new health practices finally led to a critique of the social uses of these services and to a change in their functions. As true medical expertise, regulation is an answer to the need to rationalize flows in urban space.