AT&T MERLIN LEGEND Release 4.0 Feature Reference page 126

Feature Reference
Analog Multiline Telephones
On analog multiline telephones, special ringing patterns differentiate various call
types. If Personalized Ringing is used, the personalized ring comes after the
distinctive pattern.
Centrex intercom calls are indicated by a beep followed by the
personalized ring.
Centrex special signaling is indicated by the facility-tracking tone.
Centrex special or priority calls are indicated by two short rings followed
by the personalized ring.
Outside calls are indicated by one short ring followed by the
personalized ring.
Single-Line Telephones
When single-line telephones are used in Behind Switch mode, a prime line is
assigned automatically to the extension.
Centrex service supports only touch-tone telephones.
When single-line telephones are connected directly to a prime line, they have
limited functionality because they cannot access communications system
features or make intercom calls. They can, however, use all the Centrex features
by dialing the proper access codes.
If a single-line telephone has the Idle Line Preference programmed for an ICOM
Ring button, the user has complete use of all communications system features.
Access to Centrex lines and features is gained by dialing the Centrex access
code. However, a single-line telephone cannot use the communications
system's Conference, Transfer, or Drop because the switchhook flash goes
directly to the Centrex line and is not intercepted or interpreted by the
communications system.
Single-line telephones should be connected using an 012, 016, or OPT module.
If a single-line telephone is connected to a MFM, it cannot send a switchhook
flash.
In Hybrid/PBX mode, special ringing patterns are used on single-line telephones
to differentiate various call types. (Personalized Ringing is not available.)
Centrex intercom calls are indicated by 2-burst ringing.
Centrex special or priority calls are indicated by 3-burst ringing.
Outside calls are indicated by 3-burst ringing.
Centrex special signaling is not indicated.
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Centrex Operation