The European market for roaming services has historically experienced poor or no competition and, when competition has occurred, prices have been largely resistant to its downward pressure. The root cause of this failure lies in the technical limitations of the existing roaming architecture. An architecture able to remove these limitations should allow the contractual mobility of roaming users: users should be able to select their access provider automatically, on a per-session basis and on one-to-one contractual basis. In this paper, we study the extent to which, by removing these constraints, the enablement of contractual mobility influences user and provider behaviour, creating an improved market outcome. To this end, we develop a model of the relationships between contractual mobility as a design choice for roaming, the behaviour of market players in response to that choice, and market outcomes. We then study its evolution over time as design choices and settings change in different experiment scenarios. Our results show that enabling contractual mobility - either as a commercial choice by providers or mandated by the regulator - creates similar incentives for both potential users and new providers to enter the market, while decreasing switching barriers. These encouraging, albeit seminal results could form the basis for the development of a techno-economic model of roaming aimed to reduce the uncertainty as to the timeliness and efficacy of national regulators' policies.