If it's real high demand, online is your only option. It will be gone by the time the call center opens.
Something else to consider. If you are wanting the week of Christmas, people will book the weekend before Christmas early by booking a split reservation. For example, let's say Christmas fell on a Wednesday, and the reservation you'd really like to have is from Saturday the 21st to Saturday the 28th. You might wait until Feb 21st to book your week, but find that there is no availability on the 21st. Why? Because people already booked a split week that used all the first part of that week. The earliest you can book to get a week that starts on Dec 21st is actually Feb 17th, you will look for a 4/3 split. You can check on the 21st to see if the full week you want is there. If it isn't, there is a good chance that again at 7:00AM on the 24th, you'll be able to pick up the rest of the week you want. The caution there is that people will also be trying to book early for the week between Christmas and New Years, and unlike the week prior to Christmas, all of the days of that week are wanted. So you're competing against two groups - people completing their weeks for Christmas week, and early bookers for the week between Christmas and New Years. Fun!