Panasonic KX-TD500 System Reference Manual page 1451

3.30 HDLC failure (ATLC card)
Error Code
Possible cause of
the malfunction
Countermeasures
Note
B yxx
xx= 01 to 40 : Slot number
y= 1 to 2 : Port number
1) Defective HDLC IC
Set the faulty port to OUS, then
enter the port test command
Faulty
Replace the faulty ATLC card
None
15-E-33
End
Pass