When setting up a conference call, one participant acts as the
    operator. The operator establishes the conference by calling the
    other participants in turn and waiting for an answer. A maximum
    of six participants can take part in a conference call.

    Only users currently participating in the conference call can
    bring in new participants with the DIAL or PHONE command. If
    you dial a person already participating in an ongoing conference
    call, you talk only to the participant you called.