AT&T MERLIN LEGEND Release 4.0 Feature Reference page 124

Feature Reference
automatically reassigned and their removal leaves empty places in the
sequential numbering of extensions. See "System Numbering" for additional
information.
Beginning with Release 3.0, companies may use the 800 GS/LS-ID module to
capture calling number identification information (subscribed to from the CO on
loop-start lines only, if available) and MLX display telephones in these systems
to show the number of an outside call received on a line connected to the
module. However, if the customer also subscribes to call waiting through
Centrex, the number of the waiting call is not shown on the MLX display. For
more information, see "Caller ID."
Mode Differences
Hybrid/PBX Mode
Hybrid/PBX mode can only be used in a limited Centrex configuration. Prior to
Release 2.0, a switchhook flash could be sent to the Centrex service only when
the prime line was terminated on a personal line or Pool button. (Prime lines can
be shared.) Accessing the same prime line through an SA button did not allow
the switchhook flash to be sent to the Centrex service.
In Release 2.0 and later, Centrex lines active on an SA button (including a
Shared SA button) can use Recall or switchhook flash.
Tie, WATS, and T1 lines can be used in pools. They can only be used as
personal lines with Centrex service in Key and Behind Switch modes.
Key Mode
Key mode can only be used in a limited Centrex configuration.
Key mode avoids the problem of each extension requiring a prime line (or
shared prime line) to make Centrex calls. It allows the use of an ICOM button for
access to Centrex lines. It also allows the use of tie, WATS, and T1 lines as
personal lines.
In releases prior to Release 2.0, a switchhook flash can be sent to the Centrex
service only when the line is terminated on a personal line. Accessing the same
line through an ICOM button does not allow the switchhook flash to be sent to
the Centrex service.
In Release 2.0 and later, Centrex lines active on an ICOM button can use Recall
or switchhook flash.
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Centrex Operation