Panasonic KX-TD500 Installation Manual page 260

Modem failure (RMT/ERMT card)
Error Code 1304 wxxy
w= 1 to 3 : Shelf number
xx= 01 to 14 : Slot number
y= Error type
Possible cause of 1.
the malfunction
Countermeasures
Note None
Troubleshooting
Modem failure
Replace the card.
End
5.2 Troubleshooting Guide
261