Northern Telecom 3X8 Owner's Handbook Manual page 166

Notes
You can make a Priority Call only while your telephone display
shows one of the following prompts:
If Call Forward is active at the telephone you are trying to reach,
your call will be forwarded.
If the telephone receiving the Priority Call is in a conference call,
the other two parties are automatically put on hold when the
Priority Call is accepted.
Permission to make Priority Calls from a telephone is given in
Administration programming.
PO84262701
Priority Call
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DR5.1 Owner's Handbook