+1.800.247.3247 (Internal US Toll Free number, so free call on Skype)
Hours: Dallas, TX / US Central Time
M-F 0700-2230 CT
S&S 0700-2000 CT

I'm pretty sure they can Waitlist, but after it's ticketed. Best thing is to ask during the booking process. In my 20 years of flying xONEn tickets, there have only been two times I might have needed to be waitlisted. The most recent was when I rescheduled EU-NRT flight on a 3 years old AONE3 due to Covid delays, and I was booked into D instead of A and they said once it was ticketed I could call back and have it waitlisted for A class. Since this was the last flight of my AONE3 and due to strict Covid rules, I didn't plan on flying this final segment so actually never waitlisted the A class. The other time was a LHR-MLA flight that was dropped, and had to rebook to a LGW-MLA flight and there was no D class, so I just flew L in an exit row seat so didn't even bother with checking about being waitlisted.