AT&T MERLIN LEGEND Release 2.0 Equipment And Operations Manual page 209

Applications
In Release 2.0, a timed flash can be generated on a call terminating on a
System Access button, as well as on a DFT button or a DPT button. This
includes, among others, transferred calls, group calling calls, and forwarded
calls.
The following apply to timed flash (Release 2.0):
Dial Access to Pools: If the Recall button is pressed during dialing
while connected to a trunk or when end-of-dial is reached, a timed flash
is generated, the accessed trunk is kept, and restrictions are applied.
Automatic Route Selection (ARS): While an ARS call is being dialed, a
timed flash cannot be generated. When dialing is complete, pressing
the Recall button generates a timed flash, the accessed trunk is kept,
and restrictions are applied,
Rotary trunks: A timed flash cannot be generated during dialing. When
dialing is completed, pressing the Recall button generates a timed flash,
the accessed trunk is kept, and restrictions are applied.
Full Centrex Service
With full Centrex service, each telephone has a direct line to the CO Centrex.
The direct line allows users to dial outside numbers directly after dialing an
access code (usually a 9). The direct line is also used to call other four-digit
Centrex extension numbers. System intercom lines are used to dial other
telephones on the system.
Users with full Centrex service can send a switchhook flash via the Recall
button without the system intercepting or responding to the signals,
A full Centrex service requires that the planning form for each MLX telephone
using a direct line be marked for central administration of a Recall button (code
*775).
Limited Centrex Service
Limited Centrex service is for customers who will primarily use the system
features, but wish to retain access to the network or other Centrex locations by
use of a limited number of lines.
With limited Centrex service, some telephones have direct Centrex lines, while
others do not. Some telephones may be assigned POTS, Tie, or DID lines.
Others will use System Access buttons to access pooled facilities. Generally,
users will rely heavily on the features of the system.
In a limited Centrex configuration, the system provides the primary connection
to the CO, serving as a "local" switch between the telephones and the CO, A
switchhook flash, feature access code, or feature button signal is interpreted by
the system to be a system command, not a Centrex command.
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Centrex Operation