How to add my phone number back to my account

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

You will have to use the other options, such as alternative email address, authentication app or passkey.