NEC Xn120 Getting Started Manual page 41

When Power Fail is set to ON (marked PF on the card):
When the power fails (or the Xn120 is switched off) Exchange line CO1 is automatically connected to the
normal telephone plugged into socket ST8.
When the Xn120 is powered on the telephone in socket ST8 will operate as a normal Xn120 telephone.
You can not use a Xn120 system phone in socket ST8 with the power fail link set to PF.
When Power Fail is set to OFF (marked KT on the card):
When the power fails (or the Xn120 is switched off) Exchange line CO1 is not automatically connected to
socket ST8.
When the Xn120 is powered on the telephone in socket ST8 will operate as a either a normal Xn120 telephone
or Xn120 system phone, depending on which type is plugged in.
You can use a Xn120 system phone in socket ST8 with the power fail link set to KT.
Xn120 Getting Started Guide
Configure the Xn120
Power fail
ON
Power fail
OFF
41