C H A P T E R 24 Customer-Premise Switching System Features

Key Telephone System Features
At one time key systems were electromechanical devices that had a limited set of
features. New electronic systems have far surpassed these limitations, but the features
of the last generation of electromechanical key systems defined the way users expect key telephones to work. The principal features are designated with
illuminated buttons. A dark button indicates an idle line and a solid light denotes
that the line is in use. A slow flash indicates an incoming call and a fast flash
shows that the line is on hold. These buttons and lamps define the following
features, which are common to all key systems:
_ Call pickup: Any station can access a line by pressing a line button.
_ Call hold: A hold button (usually red) can be pressed to hold the line
in the central unit. By contrast, the hold button on a POTS phone holds
the line in the telephone so the line cannot be used for another call.
_ Intercom: Acommon path shared by all telephones is used to announce calls.
_ Supervisory signals: Lamps show when a line is ringing, in use, or on hold.
_ Common bell: A bell common to all lines signals an incoming call. A slow
lamp flash shows which line is ringing.
The central control unit is known as a key system unit or KSU. Electronic
KSUs can support many additional features that are characteristic of most key systems.
The list below is in addition to the telephone set features such as last number
redial, message-waiting lamps, speakerphone, call logging, speed dial, etc.
In addition to the flashing lamp call indications, call status information may be
displayed on the telephone.
_ Automatic line selection: When the user picks up the phone, an outgoing
line is selected automatically.
_ Bridged call appearance: The same extension number can be terminated
on multiple phone sets.
_ Call drop: A call can be terminated without hanging up the receiver.
_ Call forwarding: Users can forward their calls to another station
in the system.
_ Call park: This feature places a call in a parking orbit so it can be
retrieved from any telephone in the system.
_ Call transfer: An incoming or outgoing call can be transferred
to another user.
_ Callback: If someone transfers a call to an extension that does not answer
after a set number of rings, the call returns to the original station.
_ Camp-on: Users or the attendant can send an external call to another
telephone even if it is busy. The callee hears a faint camp-on tone.
When the user hangs up, the camped-on call rings at the station.
_ Conferencing: Stations can bridge two or more lines together for
a multiparty conversation.
_ Distinctive ringing: Different ringing tones enable users to distinguish
between internal and PSTN calls.