Panasonic KX-TG313SK Operating Instructions Manual page 16

Making/Answering Calls
Call share
You can join an existing outside call.
To join the conversation, press M
the other handset is on an outside call.
Note:
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
privacy mode on (page 20).
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