This changer has been designed for installations where internal communication between several telephones / monitors is required, as well as their communication with the entrance panel, that is, for installations with internal communication (between telephones / monitors) and exterior (with entrance panel). This changer works exclusively with Palace BUS3 Telephones Ref. 8084/8086 and Palace BUS3 Monitors Ref. 1221. Up to 12 Telephones / Monitors are allowed per dwelling, 6 "main" and 6 "secondary". Communications are always secret. Only the entrance panel and a housing terminal (telephone / monitor) or two interior terminals (telephones or monitors) may be in communication. The ringtones are different according to their origin, interior or exterior. It is possible to restrict the communications between secondary telephones / monitors, that is to say that calls cannot be made between them.