A network communications system comprises a number of user stations, coupled via a data network to a number of service provider host systems. The user stations are registered as client stations at the host system of a service provider, with the host system maintaining a database of registered client station identification data. The host system additionally maintains a directory of the client stations that are on-line at a given time, determined from periodic messages sent to the host by each client station. Callers wishing to contact a client station (10) user first contact the service provider which, from the directory, determines whether the user is currently on-line. If so, a paging message is generated by the service provider from data supplied by the caller and sent via the network to the client station, the address of which is held in the service provider database.