Several ISDN telephone terminals in a passive bus configuration are enabled to be used to share a single voice communication in the familiar extension phone arrangement by means of a second bus connecting the analog sides of the terminal circuits. Once an incoming or outgoing voice call has been established at one of the number of ISDN terminals located on the premises, any others of such terminals may join the conversation by simply completing an off-hook connection by way of the analog bus. In conversion of premises to ISDN systems, the abandoned conventional telephone wiring may be employed as the analog bus. Conventional analog phones may also be utilized with ISDN voice terminals via the analog bus.