A system and method for determining the availability of a mobile communications device on a network based on a prediction that the device will cross a peer-defined service boundary. The service boundary may be the periphery of a region in which the received signal strength is stronger than a minimum value, or the periphery of a region in which a particular service provider's signal is dominant. When the device is about to cross the service boundary, or has crossed the boundary, a notification of a change in service is issued to at least one of the user of the device, a presence service, or another network operator. The notification may comprise a change in the presence status of the mobile communications device. The service boundary is defined using aggregated data from a plurality of mobile communications devices reporting the received signal strength at various geographic locations.