For future visitors searching this thread, rtw's cannot be done by any agent that answers the phone, you need to get the rtw desk to which any (LH) agent should be able to transfer you. Other carriers have rtw or *A desks that could potentially book on their ticket stock. Your choice of ticket stock can impact availability if you expect to make changes in the future (e.g, per another thread 016 UA ticket stock may not be able to access SQ D even when fully available).