This paper analyses from both an institutional and strategic point of view the market of telecommunications' directory services, i.e. the "White and Yellow Pages" directories, that has been recently liberalised due to the European Directive 19/96/CE. The theoretical background of the analysis is related to the concept of two-sided markets, i.e. markets involving two groups of participants (buyers - consumers and sellers - retailers) who need to interact via competing or monopolistic intermediaries. The Telecoms' directory services can be classified as a network good since the value of the product strictly depends (indirectly) on how many consumers use the product: consumers value directories based on how much information and advertising are in a directory, while retailers place more advertising in a directory according to the number of consumers of a directory. The analysis points out the features of natural monopoly that characterised the market and the effects of competition between intermediaries. The results show that competition is not preferable tout court and so that policy makers (governments, regulatory or antitrust authority) must treat the market opening issue carefully.