Managing personal Outlook.com account settings, security, and privacy
Hi. I'm Brian and I'll try to help.
When you plan to change phones, always port your number to the new phone, if possible, so you can always get messages. Also, you must back up your authenticator secrets so that after installing the authenticator on the new phone, you can import those secrets to the authenticator again. Also a possibility, keep the old phone, which may still be able to access the accounts because of the stored secrets, so you can disable multifactor authentication using that phone, install the authenticator on the new phone, then re-add the secret using the authenticator on the new phone. When you enable multifactor authentication, you become solely responsible for maintaining access to your account. Unless you are a member of an organizational Microsoft 365 tenant, no one can disable multifactor for your account. It's gone. That's the whole point of multifactor authentication.