Understood, but the underlying problem may be the same. Capital One wants to use your cellular account information to verify your identity. If your phone account is prepaid or through a reseller, T-Mobile may not be able to provide the service. I think you will need to contact Capital One again, on a weekday during normal business hours, get past the front line staff, until you can speak with someone who knows exactly how the system works.
Well thanks, but my phone is direct through t-mobile and there is no reseller etc.
CapitalOne "Digital" department say they are still working on the bug with no ETA.