Panasonic KX-TMC40JX-W Service Manual page 33

Call Waiting
Calling Identity Delivery on Call Waiting (CIDCW) is a CLASS service that allows a customer,
while off-hook on an existing call, to receive information about a calling party on a waited call.
The transmission of the calling information takes place almost immediately after the customer
is alerted to the new call so he/she can use this information to decide whether to take the new
call.
Function:
The CAS signal flows from T/R C181, C182 R181, R182 IC201 pins 38~40 and the ACK
(DTMFsignal) is returned to the telephone exchange. The telephone exchange that received the
ACK can send the data in the same manner as the callerID. If the unit deems that a telephone
connected in parallel is in use, ACK is not returned even if CAS is received, and the information
for the second and subsequent callers is not displayed on the portable handset display.