obviously conference is not working properly.
When being in a phone call and adding an additional attendee
a separate entry in the dialog is displayed and the partner is ringing.
But, ringing can not be stopped nor the attendee removed from the list
as it is not shown in the conference list. Even worse, when the attendee
does not take the phone call and connection is dropped, jitsi keeps ringing.
Then the only solution is to stop the whole conversation and start anew