The current invention relates to the registration of mobile stations in a mobile communications network. It has particular relevance to handovers and roaming between base stations of an unlicensed radio access network (110) and those of a conventional cellular network (120). As the cells in an unlicensed radio access network have a rather small size, it means that handover potentially can occur far more frequently than for a conventional cellular network. This can result in an undesired ‘ping-pong’ effect, i.e. that the mobile station is switching back and forth between two radio cells too frequently. The current invention reduces this effect by preventing the mobile station to register to the unlicensed radio access network during a modifiable period of time (306).