AT&T MERLIN LEGEND System Reference Manual page 312

Features
Key and Behind Switch
Considerations
Feature Interactions
Alarm
In the Key and Behind Switch modes, when Ringing/Idle Line Preference is
enabled and Automatic Line Selection is set for an Intercom button, the single-
line user can select the personal line to make an outside call by dialing the Idle
Line Preference code (usually 9) while listening to the inside dial tone.
A maximum of 64 stations can share a personal line.
If a non-DID line/trunk is not assigned as a personal line, nor grouped in a
pool (Hybrid/PBX only), nor assigned to ring into the QCC queue
(Hybrid/PBX only), and a call is received on the line/trunk, the caller hears
ringback even if that line/trunk does not terminate anywhere in the system.
If a DID trunk is assigned as a personal line, and a call received on the DID
trunk is ringing at the station programmed to receive the calls (the routing
station), the call can be answered by using the Personal Line button.
However, this is not recommended since the purpose of DID trunks is to
route calls to specific stations without the need for personal line assignment
or system operator assistance.
A station can be programmed as the principal user (owner) of a personal
line. When this is programmed, the extension can forward calls to an outside
number by using Remote Call Forwarding. When the owner has Individual or
Group Coverage, calls received on the personal line follow the owner's
coverage and not the coverage of telephones that also share the personal
line.
When no principal user is assigned for a personal line, calls received on the
personal line are not forwarded to outside telephone numbers but do follow
the Coverage patterns for all users who share the line.
Two users can join an in-progress call (including conference calls) for a
maximum of three users on the same call.
Outside lines/trunks used as personal lines cannot be assigned to a pool and
cannot be assigned as Loudspeaker Paging, Music-on-Hold, or Maintenance
Alarm ports.
DID trunks (Hybrid/PBX only) cannot be used as personal lines.
Personal lines cannot be assigned to a QCC(Hybrid/PBX only).
A line/trunk jack used for a Maintenance Alarm cannot be assigned as a
personal line.
Personal Lines 2-263