AT&T MERLIN LEGEND Release 4.0 Feature Reference page 123

Feature Reference
With full Centrex, the Recall or Flash and fixed-function buttons (Conf,
Transfer, and Drop) control Centrex functions. Corresponding communications
system functions can be programmed on buttons if any are available. (See
"Recall/Timed Flash" for additional information.) With limited Centrex, the Recall
or Flash and fixed-function buttons control communications system functions. In
either case, some Centrex functions can be programmed on the Directory and
on Auto Dial buttons, but not on other unused feature buttons.
Centrex service is supported only on loop-start trunks. A new tariff allows central
offices to offer Centrex features on ground-start trunks; however, the MERLIN
LEGEND Communications System does not support Centrex features on
ground-start trunks. Centrex service on T1 trunks with loop-start emulation is
also not supported.
During high-traffic periods, the loop-start lines/trunk used by Centrex can cause
a glare problem when multiple calls access the same line simultaneously.
Loop-start lines/trunks also have higher cable losses than ground-start lines and
cannot guarantee secure toll restriction.
With limited Centrex in Hybrid/PBX mode, DID, tie, WATS, and T1 lines/trunks
can be used. In Key mode, tie, WATS, and T1 lines/trunks can be used. These
lines/trunks cannot be used with full Centrex in Behind Switch mode.
With limited Centrex, outside calls made by using an SA button to access a pool
require two access codes for outside calls, one access code for the pool, and
one for outside lines on the Centrex service.
Centrex users should not be assigned calling restrictions because the system
prevents a telephone with calling restrictions from sending a switchhook flash to
the central office. Calling restrictions should be placed through the Centrex
service.
Once a call connection is made to Centrex service, the communications system
cannot detect additional calls that are initiated following a Centrex switchhook
flash. Therefore, the SMDR and systems such as Call Accounting System (CAS),
Integrated Solution II (IS II), Integrated Solution III (IS III), and Call Accounting
Terminal (CAT) do not report the additional calls.
Users who have access to both Centrex and communications system features
must be aware of which they are connected to when they attempt to use a
feature. Use of Centrex buttons when connected to the communications system,
or of communications system buttons when connected to Centrex service,
causes misdialed calls.
If a Multi-Function Module (MFM) is not being used on an MLX telephone, the
second extension should be removed, in order to reduce the number of Centrex
lines. The automatic assignment of two extensions to each MLX set may mean
the installer must renumber the system, because the removed numbers are not
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Centrex Operation