Panasonic KX-T336 System Reference Manual page 1357

Testing sequence after dialing "90" is given below by using an example of PITS model
KX-T7130.
In LED test, " " means the light off, "
means flashing.
LED test
1) Testing the DN button indicators and the INTERCOM
button indicator for lighting in green.
INTERCOM
buttons
2) Testing the DN button indicators and the ICM button
indicator for flashing in green.
INTERCOM
buttons
" means the light on, "
LED TEST
LED TEST
12-C-30
(70695)
"
DN buttons
DN buttons