Thank you, everyone. Problem solved.

@Pete ****

ok, Pete...you win. "Clunky" was the problem.

@Pablo *******

I tried all sorts of variety of numbers to see if it was just looking for a different number of digits...no luck. I don't know why it thinks it knows better than I what is my mobile number :-(

@Pablo *******

or did you mean to drop the "1", which I'd already done (I don't really consider that part of the number)

even though there is already a dropdown for the country code? So, put "001" then drop the first digit of the area code?

@Scott *******

I probably should have been clearer in my post. I will fix it now. I meant people who still have a U.S. phone number. That seems to be what's causing my problem, although I didn't try this site. Third party sites don't generally allow one to check seating availability prior to booking which is rather critical for me.

@Pete ****

sorry...five valid phone numbers, actually

@Pete ****

I'm not sure clunky explains not accepting a valid phone number (or rather four valid phone numbers).

@Mika *******

that does not solve my problem. First of all, I'm not permanently here, so there's no reason to do so. Secondly, I still need to enter an emergency contact. My emergency contact is not here. As I said, on one of my attempts, it actually accepted my number but not my daughter's. No matter what my SIM is, my daughter is still not in Thailand. I'm not sure why I need to pickup a SIM just to bypass one field on the AirAsia site when it doesn't solve my problem.

I'm not getting a seat assignment option :-(. I'm going to try one more time with an upgraded ticket (it didn't say seat assignment was part of an upgrade but it usually is).