Online opens one hour ahead of the phones. You should always try the website or the app right when things flip at 6am ET if you're trying for anything difficult. If there are glitches, keep trying until 7am when the phones open. That's the only time I bother calling. It's come up many times and the reservationists will tell you that they are using the same exact system, so they have no great advantage over anyone doing this online or on the app. In fact, I can search the online system or the app so much faster than anything can be done by phone, so I really just see the phone as an "if all else fails" sort of thing.