AT&T MERLIN LEGEND Release 4.0 Feature Reference page 119

Feature Reference
Additional features, such as speed dialing and night service, may also be
available from some telephone companies. Centrex features other than those
specifically discussed in this section are accessed by sending a switchhook
flash and dialing the appropriate feature code. These codes are not intercepted
or interpreted by the communications system.
To use the features available through Centrex, dial a Centrex feature code from
a touch-tone telephone. Some features must be programmed for customers by
the telephone company at the central office (CO). The system can be
configured for either full or limited Centrex service.
Full Centrex
Full Centrex requires that each telephone have a direct Centrex line/trunk ( prime
line ) to the CO. Full Centrex can also be used when only some telephones have
prime lines, but the telephones without prime lines have limited ability to use
Centrex features. Prime lines can be shared between telephones.
The prime line allows users to dial outside numbers directly after dialing an
access code (usually
telephone must be programmed by the telephone company.
The prime line is also used to call other 4-digit Centrex extension numbers that
may be located at different sites served by the same telephone company. The
communications system's intercom lines are used to dial other telephones in the
communications system.
With full Centrex, users can send a switchhook flash using the Recall or Flash
button. The fixed-function buttons (Hold, Drop, and Transfer) control Centrex
features rather than communications system features. (Additional buttons can
be programmed for communications system use.) The communications system
does not intercept or respond to Recall or fixed-function button signals. See
"Recall/Timed Flash" for additional information.
For full Centrex operation, the system must be in Behind Switch mode. A full
Centrex configuration operates on three levels, as shown in Figure 4. The
telephone user must be aware of which level he or she is at when making a call
or activating a telephone feature.
). For this reason, any calling restrictions for the

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Centrex Operation