AT&T MERLIN LEGEND System Reference Manual page 524

Hardware
* One or two DSS adjuncts can be added to provide 50 or 100 additional extension buttons.
5-18 Telephone and Consoles
System operator functions are provided via DLCs using the following
telephones:
MLX-20L*
MLX-28D*
MERLIN II System Display Console with built-in DSS
BIS-34D
BIS-34
BIS-22D
Note: In a Hybrid/PBX system, the MLX20L telephone can function as a QCC; it
is the only telephone that can do this.
DLCs are connected either to an analog station jack on a 008 or 408 analog
multiline telephone module, or to a digital station jack on a 008 MLX module.
QCCs must be connected to a digital station jack on a 008 MLX module. The
first QCC must be connected to the first station jack in the system.