Panasonic KX-TD500 System Reference Manual page 1538

Conditions
SLT telephones and some PITS telephones* can
be used during power failure if power failure
transfer assignment had been done in advance.
*Following PITS telephones can be used during
power failure.
PITS-KX-T123230, KX-T123230D,
KX-T123235, KX-T61630, KX-T30830
When you are using the PITS telephones avail-
able with power failure transfer, set the POWER
FAILURE switch to ON, when power failure
occurs.
If dialing cannot be done, set the DIALING MODE
selector to another position (PULSE or TONE).
When the power is restored, set the POWER
FAILURE switch to OFF.
If the power is restored during a conversation, set
the POWER FAILURE switch to OFF after
conversation is completed.
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