Update: Figured out the problem when I called HAL (only waiting for a couple minutes on hold! Woo hoo!!). I was already booked for an excursion at the same time as the waitlisted one that became available and that I was trying to book. It would have been nice to get a message that indicated that, rather than the "please add your telephone number" message that I got, but, of course, I could have checked my itinerary and realized there was a conflict. Anyway ... all taken care of. Really pleased that my phone call was answered so quickly and that the customer service person was very helpful!