This article analyses the social and legal construction of an interactive advertising project. The project consisted in the transmission of multimedia content, via Bluetooth, to the mobile phones of users at stations in the Paris underground. In this case the legal qualification of the Bluetooth identifier and the modalities for obtaining users' consent to receive a commercial call are important and contested issues. They reveal the contrasting positions of actors from the technical, legal and commercial worlds, as well as the compromises that were reached as the project advanced, up to its submittal to the CNIL* for assessment with respect to potential privacy threats. This process of configuration, leading to a process of construction, is generally neglected in studies on uses, which all too often consider the environment as "given". The project studied here exemplifies a more general tension between two different conceptions of the accessibility of mobile passers-by in urban public spaces, which also paint different pictures of urbanity and the public sphere: accessibility based on proximity, and accessibility based on connectivity and reachability through mediated communication.