If you want another not quick but compliance approach to sort this out, email [email protected] (their data protection team). As a data subject you have the right to be able to ensure that the data a company holds about you is accurate, and if customer service is unable to update a phone number for 2FA purposes that is either a security process failure or incompetence. Anyway, the data protection team would probably like to know about this as they can go an bang their compliance hammer internally, and they might help you fix your duff phone number if you can prove to them who you are to their satisfaction.