Geographically distributed conference sites each have a telephone receiver and a dataconferencing appliance connected to a shared voice call network. Each site also has a display device for displaying images comprised of image data produced by an image source. The dataconferencing appliance is connected to a data network that links the sites independently of the voice call network. To initiate a dataconference session between the sites, a voice call session is first established between the telephone receivers via the voice call network. An address negotiation procedure implemented in software or hardware of the dataconferencing appliance is then manually initiated to identify a network access code and communicate it to at least a second one of the sites within the voice call session. The network access code is used to establish a data communication session via the data network for convenient transmission of image data between the sites.