About Directory Numbers (IM: Telephony)

Inside Macintosh: Telephony /: Chapter 3 - Directory Numbers

About Directory Numbers
A telephone terminal is associated with at least one directory number, a named reference point that is used to initiate or receive calls on the terminal. A terminal can have multiple directory numbers (just as a typical telephone set in an office might have multiple buttons). A directory number can, in turn, have multiple network subaddresses.
A directory number might support some or all of these capabilities:

The Call Forwarding feature. Incoming call appearances on one directory number are forwarded to some other directory number.
The Do Not Disturb feature. An incoming call appearance of a specific type is prevented from alerting on a directory number.
The Message Waiting feature. The Telephone Manager records a voice message from an incoming call appearance on a directory number when that directory number is already active with an existing call appearance.
The Paging feature. An incoming call appearance causes a user to be paged.
The Intercom feature. The telephone network supports one or more intercom operations.
The Directory Number Select feature. The directory number can be selected from among a set of directory numbers associated with a telephone terminal. Selecting a directory number puts on hold all calls of the previously selected directory number. Deselecting a directory number drops all call appearances associated with that directory number
The Call Pickup feature. A call appearance alerting at one directory number can be answered at another directory number.
The Auto-answer feature. An incoming call appearance can be automatically routed to an application that has previously requested notification of incoming calls.
The Call Saver feature. An incoming call appearance can be answered after fewer rings than normal if messages are pending for the directory number.

A feature is subscribed if the user's telephone network switch provides it and if the terminal supports it. A feature is available if the directory number of interest is in a state in which using the feature is valid. A feature is active if the feature is currently in use by the directory number.