I had a similar problem with reaching the box office. However, I got through when I called the main number from the Drury Lane website.
The box office is generally open the day of the show at 9:00 a.m. I would keep trying to call. I also know that you used to be able to leave a message not sure about that now
I believe they are only open all day on Tuesdays. Since your date is tomorrow, I think you are out of luck. Try again after 4pm. May be open if there is an event.
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