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As the designs of private automatic branch exchanges (PABX's) evolve towards "electronic offices" and incorporate aspects of the integrated services digital network (ISDN), public exchanges are themselves taking on functions that were often only found in private networks. It is therefore pertinent to question the status of the PABX as a node of the ISDN rather than as a mere customer terminal. With the present designs of PABX's, public exchanges, and the signaling systems employed, it is not possible to provide PABX extensions with all the facilities on the public network which are available to directly connected main-exchange subscribers, although facilities available over private networks continue to proliferate. This paper considers aspects of signaling via both public and private networks to ensure that there need be no restriction on the facilities offered.