Panasonic KX-TD500 System Reference Manual page 1196

Conditions
Some switching system of central office may send CPC-like signal in dialing
sequence and the attempt of making a call may be terminated.
If your switching system does not send CPC-like signal in dialing sequence,
we recommend to make CPC signal detection work on outgoing CO calls.
CPC signal detection can be assigned to incoming CO calls only or both on
incoming and outgoing CO calls. If CPC signal detection is assigned to
outgoing CO calls only, it does not function.
11-C-70