Panasonic KX-TG6672B - Manual (Page 16)

Call share
You can join an existing outside call.
Base unit
To join the conversation, lift the corded handset
when the handset is on an outside call.
Handset
To join the conversation, press M N when the
other unit is on an outside call.
Note for base unit and handset:
R A maximum of 4 parties (including 1 outside
party) can join a conversation using 3
extensions. (4-way conference)
R To prevent other users from joining your
conversations with outside callers, turn call
sharing mode off (page 23).
16 For assistance, please visit http://www.panasonic.com/help
Making/Answering Calls