NEC Electra Elite 48/192 Features And Specifications Manual page 467

Issue 6
P - 10
Single Line Telephones must be equipped with a ground button for
¥
use with Ground-Start Trunks.
System features cannot be activated from Single Line Telephones
¥
when Power Failure Transfer is in operation.
When power is restored to the system, Power Failure Transfer is
¥
canceled. Calls in progress on Power Failure Transfer lines are
disconnected.
Electra Elite
Power Failure Transfer