AT&T MERLIN LEGEND Release 4.0 Feature Reference page 173

Feature Reference
Pool buttons cannot be assigned on a DLC; however, trunks included in a pool
can be assigned as personal line buttons on a DLC.
Trunks that are not assigned to buttons on the DLC can be selected by the
operator only by dialing the pool dial-out code from the SA button or, on an MLX
DLC, by selecting a DSS button for the pool dial-out code.
Trunks that are not assigned to a pool cannot be selected from a DLC unless
they are assigned to buttons on the console. Shared SA buttons cannot be
assigned to DLCs.
Key and Behind Switch Modes
Only DLCs (not QCCs) are allowed in Key and Behind Switch modes.
A DLC operator cannot select lines that are not assigned to buttons on the
console.
Telephone Differences
MLX Telephones
An MLX-20L assigned as a DLC can also be used for system programming by
connecting it to any of the first five extension jacks on the first MLX module and
designating the extension jack for system programming. The Home screens of
the MLX-20L and MLX-28D are the same as that of non-operator telephones.
The built-in DSS field on the MERLIN II System Display Console corresponds to
physical extension jacks in the control unit instead of specific extension
numbers in the numbering plan. Therefore, DSS buttons on the MERLIN II
System Display Console cannot be used to monitor the busy status of trunk
pools or calling groups.
All Dial 0 calls are directed to the QCC queue and do not ring at any DLC
positions. A DLC cannot use Position Busy, which is only for QCCs. A DLC
cannot be assigned as a position-busy backup for a QCC (only calling groups
can provide backup for a QCC).
Analog Multiline Telephones
An analog DLC cannot be used for system programming.
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Direct-Line Console