Digital key telephone system

A key telephone system includes a plurality of ports linked by port associated bidirectional communication channels which are synchronously switched by transferring bit states between ones of the channels to provide communication paths between the ports as directed by a central processor. The ports are also linkable to and via the central processor by port associated message channels. An interface circuit is responsive to the central processor and message channel signals for regulating flow of messages received by the central processor and for effecting single and plural channel distribution of messages from the central processor. The message channels permit telephony operating features and functions to be provided either within the central processor or by appropriate apparatus means being connected at any of the ports. In such a telephone system at least one of said apparatus comprises a telephone station apparatus and another of said apparatus comprises a feature host apparatus. The telephone station apparatus has a plurality of functions stored therein and said feature host apparatus has a plurality of feature modules stored therein. Each such feature module comprises instructions for invoking a corresponding set of functions at said telephone station apparatus by means of an exchange of signalling and supervision messages between said feature host apparatus and said telephone station apparatus. The functions invoked provide the feature selected by a feature request action of a user at said telephone station apparatus.