Panasonic KX-TDA15 - Manual (Page 211)

4.2 Feature Number Table
User Manual 211
Feature (While busy, DND or call tone is heard) Default
1.2.4 When the Dialled Line is Busy or There is No Answer
Sending a Call Waiting Notification (Call Waiting) 1
To an Extension Refusing the Call (DND Override)
Joining an Existing Call (Executive Busy Override) 3
Leaving a Message Waiting Indication/Calling Back the Caller Who Left an
Indication (Message Waiting) 4
Monitoring Another Extension (Call Monitor) 5
Reserving a Busy Line (Automatic Callback Busy)/Setting Callback on an
ISDN Line (Completion of Calls to Busy Subscriber [CCBS]) 6
1.2.6 Alternating the Calling Method (Alternate Calling—Ring/Voice)
Feature (While dialling or talking) Fixed Number
1.4.5 Multiple Party Conversation
Adding Other Parties during a Conversation (Conference) 3
1.9.1 If a Doorphone/Door Opener is Connected
From any extension while talking to the doorphone 5