Additional features, settings, or issues not covered by specific Microsoft Teams categories
Hi Sophie,
Welcome to the community!
If you still can access the other O365 services, you can go to https://portal.office.com/account/ to update your phone number and other security methods. Click Security & Privacy > Additional security verification > Update your phone number and account security.
Change your authentication phone number and save. Or you can enable other security methods by selecting the checkbox, and use these method to verify.
Regards,
SD