The conference calling feature is useful when you're on a call and need to add another person to it. To begin the conference call, you must be on a call. (It doesn't matter if you initiated the call or if you received the call.)
You can conference up to sixteen people on your phone (including yourself).
To add people to a call in progress:
If the person you're adding cannot join the conference call, or roll to voicemail, and need to cancel the call:
Did you know? You cannot remove callers from your conference call after they're added. If a person is no longer needed, the caller must hang up. If you need to continue the call after the person has left the call, as a best-practice, you should disconnect from the current call and start the conference process again.