This issue occurs because the conference call is locked, and the organizer doesn't receive a notification that callers are waiting in the lobby. Conference calls are locked when the These people don’t have to wait in the lobby
option is set to Only me, the meeting organizer
in the Outlook invite meeting options or to The meeting organizer
in the Skype meeting options during the meeting.
For more information about Skype for Business dial-in conferencing, see the following Microsoft websites: