The best way would be to call the conference number (700 in a 3 digit PBX), create the conference room with an ID (you decide the number) and then have the other participants call 700 and have them enter the ID which was just created.
If you are just elevating a call to a conference using the phones conference facilities the initiating extension will always be needed in the conference for it to be active. In this case you arent using the 3CX conferencing facility.