Over the phone bookings in the UK do seem to have a cooling off period (haven't experienced it, only heard second hand) which I think is something to do with UK credit card consumer laws? I don't believe it applies to online bookings but I may be wrong (somebody please correct me if so).
In the US, there is no cooling off - bookings are immediately subject to the cancellation and change rules attached to the fare.
If your fare rules allow it, you may want to consider changing your booking online through Manage My Booking - change it to a future date that you might need. You can then change it again to another useful date closer to the time if necessary.
On the other hand, if the booking is of no use to you at all, you can probably process your own refund through MMB and get the maximum refund due back on your credit card as soon as possible