Panasonic KX-TDA15 User Manual page 215

Feature (While busy, DND or call tone is heard)
Joining an Existing Call (Executive Busy Override)
Leaving a Message Waiting Indication/Calling Back the Caller Who Left
an Indication (Message Waiting)
Monitoring Another Extension (Call Monitor)
Reserving a Busy Line (Automatic Callback Busy)/
back on an ISDN Line (Completion of Calls to Busy Subscriber [CCBS])
1.2.6 Alternating the Calling Method (Alternate Calling—Ring/Voice)
Feature (While dialling or talking)
1.4.5 Multiple Party Conversation
Adding Other Parties during a Conversation (Conference)
1.9.1 If a Doorphone/Door Opener is Connected
From any extension while talking to the doorphone
4.2.1 Feature Number Table
Setting Call-
Default
3
4
5
6
Fixed Number
3
5
User Manual 215