A 3 Way conference / add a caller is the ability to add another caller to the ongoing.

1. During an active call press the Add Call Button

2. The Keypad will appear.

3. Key in the number you want to join the call and press the dial button -or- select a number from your Contacts and tap/click the number.
    TIP: If you want to cancel the 3 way conference at this point tab on the top green "Dial to Transfer" and you will get the caller back. 

4. The Join Call screen will appear and you are now talking to the 3rd party, the original caller will be On Hold and hear Music On Hold

if YES the receiving party is OK to join the call press the "Join" button and the 3 Way conference will be established.
TIP: During this step you can switch between the callers by pressing the Switch (B) button. 

If NO the receiving party does not want to Join the 3 Way Conference and the 3rd party did not hang up (yet) you can hangup the call by pressing the Hangup Button (A) and then the "Hold" to take back the original caller.