To flash between calls (call waiting) just slide the green answer button on the screen to switch between the calls. There is a "switch" button on the call screen that will allow you to jump back over to the previous caller if needed.
If you're trying to do a conference call, dial the first number, then after the caller answers, press the "add call" button and dial the second party. Once they answer, press the "merge" button and you will have the conference call active.
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