Two-Factor authentication (2FA) is only available for your ZOA account if your administrator has activated the option for all users.
You change your phone number for the 2FA on the "Account security" sub-page of the "My account" page. Please click on the "Edit" button next to your registered phone number.
And add your new phone number and click "Send authentication code".
You will receive an SMS code that you need to enter in the field and click "Verify".
You will receive a confirmation that you added a new phone number.
You will also receive emails about changes to your account security.