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 .