By default if conference initiator hangs up the call, the conference is terminated.
To keep the conference parties connected when the conference initator leaves, configure under the ephone “keep-conference“. When the initiator hangsup the remaining parties stay connected. If he uses the end-call soft key, the conference is terminated.
To keep the conference parties connected when the conference initator leaves by hanging up or pressing the end-call softkey, use “keep- conference endcall” option under ephone
CCIE Voice notes 