Managing personal Outlook.com account settings, security, and privacy
Have a look at:
https://learn.microsoft.com/en-us/answers/questions/5918887/no-add-phone-number-option
Answer from Emily:
For personal Microsoft accounts, Microsoft now says you can no longer add a phone number as a new security method. Their current support article states: “For increased security, you can no longer add a phone number” when adding a new way to verify sign-in. https://support.microsoft.com/en-us/account-billing/microsoft-account-security-info-verification-codes-bf2505ca-cae5-c5b4-77d1-69d3343a5452 (see the first and last items in the gray text box). You will have to use alternative email, authentication app or passkey option instead.