AT&T MERLIN LEGEND Release 4.0 Feature Reference page 401

Feature Reference
Each pool should contain the same type of trunks (for example, basic trunks,
WATS trunks, or foreign-exchange trunks), because users cannot control the
specific trunks selected by the system. Ground-start and loop-start trunks of the
same type (for example, WATS trunks) can be mixed in the same pool. Direct
Inward Dialing (DID) trunks should not be put into pools; trunks used for Music
On Hold or maintenance alarms cannot be grouped into pools. Also, dial-in tie
trunks should not be placed in a pool that is assigned to a button on the
telephone.
Trunks assigned to pools cannot be assigned as personal lines (on buttons) on
any telephone except a Direct-Line Console (DLC). However, calls that come in
on trunks assigned to pools can be programmed to be received by one or more
QCC operators.
When all trunks in the pool are in use, the green LEDs turn on next to the Pool
buttons assigned to multiline telephones and next to any DSS (Direct Station
Selector) buttons associated with the pool dial-out code.
Individual extensions can be restricted to deny dial access to particular trunk
pools. See "Calling Restrictions" in this section.
Users with Pool buttons on their telephones can use the pool even if the pool
dial-out restriction is assigned to the extension.
One pool can be assigned to buttons on a maximum of 64 extensions.
In Release 3.1 and later, if a station is changed from a Direct-Line Console to a
Queued Call Console, pool dial-out codes are disallowed on the QCC. You must
reprogram the system if you want to allow access to dial-out codes on the QCC.
Mode Differences
Although trunk pools are available only in Hybrid/PBX mode, users operating in
Behind Switch mode can access the trunk pools in the host switch through their
prime lines.
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Pools