Configuring preferences and options in Outlook.com
Hello,
For personal Microsoft accounts, Microsoft now says you can no longer add a phone number as a new security method. This method is being phased out. See official document here.
Are you hoping to update the contact method to receive the 2FA verification?
If so, you go to https://account.live.com > Security > Manage how I sign in. And you will need to use email, the authentication app, or passkey.
If you need to verify your Bing Reward, The supported alternative to entering a phone number is QR-code verification through the Bing mobile app. Microsoft officially identifies this as an alternative to SMS, and users have successfully redeemed rewards with it when SMS verification failed. https://support.microsoft.com/en-us/accounts-billing/rewards/use-qr-code-verification-to-redeem-microsoft-rewards