AT&T MERLIN LEGEND Release 4.0 Feature Reference page 125

Feature Reference
Behind Switch Mode
For full Centrex configuration, the communications system must be in Behind
Switch mode.
Behind Switch mode does not support MERLIN MAIL, AUDIX Voice Power, Call
Accounting System, or Call Management System. These applications are
supported only in Key and Hybrid/PBX modes.
Full Centrex service supports only loop-start facilities. While lines that are not
loop-start lines will not be blocked by the communications system, they have
the potential for causing dialing errors. Even random use of modules that are not
loop-start (such as E&M modules) throws off the default line assignments. If
boards other than loop-start boards must be used, they must be positioned after
the last loop-start line module, or prime lines on later modules may be assigned
incorrectly. If a DS1 module is used, it must be placed after all other loop-start
boards on the system so that default line assignments on the communications
system are not affected. PRI facilities are not supported in Behind Switch mode.
In Behind Switch mode, during periods of high telephone traffic, users may
experience delays in obtaining dial tone from the Centrex system. This could
cause misdialing when using System or Personal Speed Dial.
Calls to calling groups in a system set up in Behind Switch mode follow the
communications system ring pattern, not the central office ring pattern.
Telephone Differences
Multiline Telephones
MLX Telephones
On MLX telephones, special ringing patterns are used to differentiate various
call types. If Personalized Ringing is used, the personalized ring comes before
the distinctive pattern.
Centrex intercom calls are indicated by the personalized ring followed by
a beep.
Centrex special or priority calls are indicated by the personalized ring
followed by three short rings.
Outside calls are indicated by the personalized ring followed by two short
rings.
Centrex special signaling is indicated by the facility-tracking tone.
Adjuncts connected to a Multi-Function Module (MFM) cannot send a
switchhook flash to the Centrex line. (Whenever possible, such adjuncts should
be attached to an 012 module or an 016 module.)
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Centrex Operation