Direct from the docs.
Conference FEATURE 3
Establish a three-party conference call.
1. Make or answer the first call.
2. Put the first call on hold.
3. Make or answer the second call.
4. After the second call is connected, press FEATURE 3.
5. Press the line or intercom button of the first held call.
6. Press RLS to end the conference call.
Variances
*(Step 5 not required)
Establish a multi-party conference call (4 or more people).
1. Follow the steps to establish a three-party conference call.
2. Press the Newcall softkey, or press FEATURE 807. You can also use the
Intercom key if your set has one.
3. After the fourth call is answered, press FEATURE 3, and press the held
line.
4. Put the fourth call on hold and repeat steps 2 and 3 to add more calls to
the conference.
5. Press RLS to end the conference call.
To remove yourself from a conference permanently (unsupervised
conference):
Press FEATURE 70.
The other two callers remain connected. (Some external lines may not
support this feature.)
To put a conference on hold:
Press HOLD. The other two callers can still talk to each other.
Telephone features
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To split a conference:
Press the line or intercom button of one caller to consult privately while the
other caller is on hold.
Note: This feature is not available for multi-party conference calls.
To re-establish the conference: Press FEATURE 3.
To disconnect one party:
1. Press the line or intercom button for the caller you want to disconnect.
2. Press RLS.
3. Press the line or intercom button to resume your conversation with the
other caller.
To independently hold two calls:
1. Press the line or intercom button of the first caller.
2. Press HOLD. The second caller is automatically put on hold.
Note: This feature is not available for multi-party conference calls.
To re-establish the conference:
1. Retrieve one call from hold.
2. Press FEATURE 3.
3. Retrieve the second call from hold.
To send Hookswitch or DTMF during a conference call:
Either system telephone engaged in a three-way conference call over a
Network CLID or DS trunk can issue a hookswitch or DTMF dialing request
without leaving the conference, if the feature is enabled.
Note: This feature is not available for IP Phones 20xx.
– To hear DTMF tones on these telephones during dial, activate Long
Tones (FEATURE 808).
– To conference in another party through the trunk, use Link (FEATURE
71)