Panasonic KX-TD500 System Reference Manual page 1448

3.27 DTMF generator failure (COT card)
Error Code
Possible cause of
the malfunction
Countermeasures
Note
80 xx
xx= 01 to 40 : Slot number
1) DTMF generator on the LCOT/GCOT card is defective.
2) DTMF signal transmission path is defective.
Set the related port to OUS
Set the smallest physical extension
number (SLC/HLC) port to OUS,
enter port test command for 2 ports
(COT-SLC)
Pass
Set the related port to INS
Confirm that a CO outgoing call
through the faulty port is possible
from a PITS telephone
OK
Replace the COT card
None
NG
End
15-E-30
Fault
Perform the same test for
other SLC/HLC cards
Fault Pass
Replace the faulty card