webpage doesn't allow adding phone number

Have a look at:

https://support.microsoft.com/en-us/accounts-billing/manage/microsoft-account-security-info-verification-codes

Note

The option to add a new phone number may not be available for your Microsoft account. You may only see options such as a recovery email address, Microsoft Authenticator, or a passkey when adding new sign-in or verification methods. Learn more about phasing out SMS as a method of authentication and account recovery.

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.

https://support.microsoft.com/accounts-billing/manage/microsoft-to-stop-sending-sms-codes-for-personal-accounts

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