AT&T MERLIN LEGEND System Reference Manual page 485

Functional Description
Hybrid/PBX
Considerations
Behind Switch Mode
4-16 Modes of Operation
Operating in the HybridPBX mode offers several advantages:
Hybrid/PBX trunks are the most efficient use of outside trunks since they can
be pooled and are more readily available to users. The ARS feature can be
programmed for more cost-effective use of pools.
Cost-effective single-line telephones can be used for greater functionality
than is possible in other modes. Both inside and outside calls can be made
by accessing a pool of trunks.
The QCC ensures efficient call handling and is particularly useful when the
number of lines exceeds the number of buttons available on a DLC system
operator position.
The system operates in the Behind Switch mode when the control unit is
connected to (is "behind") another communications system. This other system is
called the host, and can be either a PBX or a Centrex (a telephone company
service that works like a PBX but is housed at a telephone company's central
office).
Figure 4-5 shows a very simple Behind Switch setup in which the outside trunks
come into the host system.
The lines connecting the two control units are like extension numbers that allow
users to access the pool of outside lines connected to the host system. Each
extension number from the host system is assigned to individual telephones as
a Prime Line. This line rings immediately when the user receives an outside call
and the user is connected to this line (even when it is busy) unless the user
manually selects a different line.
Each Prime Line can be assigned to additional telephones as secondary lines to
allow users to see activity on other telephones sharing the button (using the
LEDs next to the button) and join their co-worker's conversations.
In addition, Prime Lines are automatically assigned to direct-line console (DLC)
system operator positions. The first line assigned is the system operator's Prime
Line and the rest are assigned as secondary lines. This allows the system
operator to answer calls received by other users on their Prime Line.