How does imposing free roaming in EU impact users and ISPs' relations?

Starting in June 2017, roaming within Europe will become free for mobile phone users, as an enforcement of the European parliament. The goal being to obtain a unified digital market: this seems at first sight to largely benefit to users, who will not have to worry about their usage abroad. We design in this paper a model analyzing the impact of free roaming enforcement on users and ISPs' pricing strategies, with respect to paid roaming to verify this assumption. We particularly focus on the strategies on transit payments between ISPs in different countries. We highlight that scrutiny is required since, depending on parameters, consumer surplus or subscription penetration are not necessarily maximized if free roaming is enforced.