A communication system is described in which mobile telephones are able to roam between neighboring home node base stations and in which the home base stations or the home base station gateway devices are arranged to aggregate “Unsuccessfully Transmitted DL Data Volume” determined by previous home base stations served by the gateway device. In this way, when a call is transferred away from the gateway device or is released, a total aggregated value of “Unsuccessfully Transmitted DL Data Volume” can be provided to the core network for billing purposes.