Voice and/or image data packets are transferred by a packet handler between user stations in a GSM-type mobile communications system using a General Packet Radio Service (GPRS) data link. Control data for controlling a call is stored in a data store accessible by the packet handler. The control data identifies call participants and the identity of a participant who has currently seized the call. A mobile station capable of video conferencing is operable in a half-duplex video conferencing mode, in which intermittent transmission of video data is controlled by depression of a transmit button.