AT&T MERLIN LEGEND System Reference Manual page 483

Functional Description
Line Access
4-14 Modes of Operation
Without additional programming, the system provides three types of trunk pools
The system's trunks are automatically assigned to the corresponding trunk type
pool. The three automatic trunk pools are
loop-start pool (also called the main pool); the factory-assigned pool number
is 70.
ground-start Pool; the factory-assigned pool number is 890.
tie pool; the factory-assigned pool number is 891.
Through system programming, the three automatic pools can be rearranged
and special-function or special-user pools can be created. For example, the
main pool can be divided and smaller pools of loop-start trunks can be assigned
to different groups of users.
Both single-line and multiline telephones are appropriate to the Hybrid/PBX
mode. In fact, the most flexible use of the single-line telephone is in the
Hybrid/PBX mode.
To make an outside call, the single-line user dials pool access or ARS dial-out
codes, and the system automatically selects an outside line. In addition, System
Access buttons on multiline telephones allow different kinds of calls to be made
from the same button (WATS or tie-trunk calls, routine outside calls, and internal
calls).
Besides System Access buttons, other types of line buttons can be assigned to
multiline telephone users. A telephone can have more than one of the same kind
of button or a mixture of buttons with the capabilities offered by the following
types of buttons:
System Access Ring buttons are used to make and receive inside and
outside calls. When a System Access Ring button is used to make an inside
call, the call rings with a one-burst ring to indicate an inside call.
System Acess Voice buttons are used to make and receive inside and
outside calls. When a System Access Voice button is used to make an inside
call, the person being called hears the caller's voice instead of hearing the
telephone ring unless the person being called does not have a
speakerphone or has disabled voice announcements.
System Access Originate Only buttons are used to make inside and
outside calls. Calls are not received on a System Access Originate Only
button. The purpose of the button is to ensure that the user always has a
button available to make a call, transfer a call, establish conference calls,
answer Call Waiting calls, or pick up parked calls. For inside calls, the button
can be programmed for either voice or ring operation.
Shared System Access buttons are used to allow two or more users to
answer each other's calls, join conversations, or make or receive inside or
outside calls on each other's System Access Voice or System Access Ring
buttons. In a Shared System Access arrangement, the principal (or primary)
station is the telephone from which specific System Access buttons (Ring,
Voice, or Originate Only) are being assigned as Shared System Access
buttons on one or more telephones.