In this article, we examine the existence of regions and borders in Belgium by means of an original data set of more than 200 million mobile communications and a mathematical method which allows networks to be divided into coherent groups in a natural and automatic manner. A geography of mobile communications is thus proposed based on the relative frequency of communications as well as their average duration. In the case of the relative frequency of communications, we obtain a map of Belgium composed of 17 groups or “telephone areas” that each contain only adjacent municipalities. On this first map, the area of Brussels is the only one which runs across the linguistic border and covers the three institutional regions of the country. In the case of the average duration of communications, we obtain a group in the north and a group in the south of the country. These two groups concentrate within themselves almost 98% of telephone calls; only 2% of communications are from one group to the other. The group in the south includes the 19 municipalities of Brussels, all of the municipalities in the Walloon region as well as eight municipalities with facilities located in the Flemish region. In particular, all of the municipalities with facilities in the outskirts of Brussels (with the exception of Wemmel) are part of the group in the south of the country. ; Dans cet article, nous explorons l’existence de régions et de frontières en Belgique au moyen d’un jeu de données original de plus de 200 millions de communications mobiles et d’une méthode mathématique qui permet de découper de manière naturelle et automatique des réseaux en groupes cohérents. Une géographie des communications mobiles est ainsi proposée sur la base de la fréquence relative des communications ainsi que de leur durée moyenne. Dans le cas de la fréquence relative des communications, nous obtenons une carte de Belgique composée de 17 groupes ou « bassins téléphoniques » spatialement bien ancrés, aux communes étonnamment toutes adjacentes. Sur cette ...