I changed carriers late last year and Capital One actually emailed me right after asking me to reverify my mobile number. Blurb from the email below. I completed the task and didn't face any issues, although I didn't try to do a transfer for a while after that. Has anyone else having these issues received this email previously?
We’ve just learned that the mobile phone number on file for you may not be correct. This could be the result of changing your phone number and/or mobile carrier. Please update your mobile number so you can receive important account alerts.
While this thread is about Capital One, I don't think this issue is specific to them. It just depends on which scenarios a bank has enabled this validation. For example, I encountered the same validation issue with Citi, although not for points transfer, instead it was when I was trying to clear a fraud block on my CC.
With Citi, removing the number entirely from my account, replacing it with another number, and then re-adding it caused something to trigger that it then validated. Attempts to just re-save the same number failed, which causes me to think that only certain steps will force an update. The check is different from traditional 2FA, as it is trying to verify the number is actually registered to you, not just that you can receive a text at the number.
This might be to save costs, as I assume carriers charge for this, or perhaps a 3rd party provider, as I can't imagine many banks are building direct integrations into every carrier.