AT&T MERLIN LEGEND Release 4.0 Feature Reference page 342

Feature Reference
Mode Differences
Hybrid/PBX Mode
In Hybrid/PBX mode, Line Request can be used for personal lines or
special-purpose lines (such as WATS) assigned to line buttons on a multiline
telephone. Callback should be used instead of Line Request to complete calls
to busy extensions or outside numbers when the call is made by using a pool in
which the trunks in the pool are busy.
NOTE:
Do not use Callback when your system includes a voice messaging system.
Key and Behind Switch Modes
Line Request only works for outside lines that are assigned to line buttons.
Telephone Differences
Queued Call Consoles
Line Request cannot be used on QCCs.
Other Multiline Telephones
Line Request cannot be used on MLC-5, MDW 9000 cordless, or MDC 9000
cordless/wireless telephones.
Single-Line Telephones
Line Request cannot be used on single-line telephones.
Feature Interactions
Callback
Camp-On
Park
Transfer
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Line Request
Returning Callback calls cancel Line Request.
Returning camped-on calls cancel Line Request.
Returning parked calls cancel Line Request.
Returning transferred calls cancel Line Request.