Well I think there's a problem that we have no proper event to cleanup the `_leftGroup` flag.
E.g. When a call left a group and keep connected for a while before disconnecting, it still brings up the status when hanged up since we clean `_leftGroup` only when it get disconnected.

I think removing `this._leftGroup = true` here resolves this problem since this is always the case of the last call leaving conference call *for now*.. but not the case if we add buttons to move a call back to normal call in the future.