How to change the "One Time Password" phone number used when I log in

I have two AWS accounts that I needed to update the phone number used for the "One Time Password" when I log in. On one account, I was able to go to the Account Settings, and see a phone number listed with the email and account name. Updating this phone number successfully changed the number that was used to verify me when I log in. On the second account, no phone number is listed in Account Settings. I've changed every phone number I can find (Contact Information and in billing) but nothing updated the OTP phone number used. Neither account used MFA.