A voice and data terminals networking system comprises a plurality of voice terminals capable of communicating with one another within an intercom system. Each voice terminal is designated with a unique first code and intercommunicated among an intercom system. The system includes a plurality of data terminals each is associated to a corresponding voice terminal and designated with a unique second code consisted by at least first and second numbers and is interconnected to a LAN system. A transition gateway selected from one of the voice and data terminals for interfacing between internal and external communications. The system further includes a device that is used for selecting the first and second codes of the voice and the associated data terminals; wherein the second number of the second code of the associated data terminal is selected by the means such that the second number of the second code is completely identical to the first code of the voice terminal. The system further provides full voice functions based on voice terminals, without the participation from the associated data terminals. The system further consists of a transition Gateway module which facilitates the flexibility of mix-and-match among WAN and LAN technologies.