AT&T MERLIN LEGEND System Reference Manual page 481

Functional Description
Hybrid/PBX Mode
4-12 Modes of Operation
A PBX (private branch exchange) originally was a large switchboard installed at
a customer's office that functioned like a small, self-contained telephone
company.
The switchboard was manually operated, and the system operators had to
physically connect calls by plugging cords into the board's jacks. Today's PBX
is a processor in the system's control unit programmed to connect both inside
and outside calls on a single button. In the Hybrid/PBX mode, this button is
called a System Access button (labeled as SA).
Although there is no longer a person handling cords, the communications
system operating as a PBX still requires the user to request a line. A user simply
dials a dial-out code (usually a 9) and the telephone number on a System
Access button, and the system routes the call to an available outside line.
The major distinction of the Hybrid/PBX mode, therefore, is that both inside and
outside calls can be made from the same button.
In the Hybrid/PBX mode, the telephone company's lines connected to the
control unit are called "trunks." The Hybrid/PBX mode can accommodate any
one, or a combination, of these trunks:
loop-start (basic lines, WATS, FX)
ground-start (basic lines, WATS, FX)
tie
DID
DS1 Facility
In the Hybrid/PBX mode, the system has the capacity to hold 11 separate trunk
pools. The number of pools programmed in the system depends on both the
kinds of trunks and the special needs of the users.
To program the Hybrid/PBX mode, the Hybrid/PBX choice on the system
programming menu is selected. Choosing Hybrid/PBX automatically arranges
the outside trunks in functional groups, or pools, within the control unit (see
Figure 4-4).
Since the outside trunks are pooled, outside numbers are not associated with
individual telephones. When a pool is assigned to a line button during system
programming, it is called a pool button. Users request specific trunk pools by
dialing the dial-out code (usually a 9) for the pool or pressing a pool button,
which gives on-touch access to a group of trunks.