After I send a fax the status is always 'Queued' even after delivery

This is weird. And in the title you wrote 'even after delivery' while in the question you wrote 'even after it has been submitted'.

Is this in the production or sandbox environment? Is the fax status still in 'Queued' now?

Thanks, in this case it turns out it is a problem with response signals being sent after the fax arrives from RC.