Is it actually wanting that phone number, or is it asking for an verification code being sent to that number?
If you do not have access to a two-factor authentication trusted number, read about how to get a verification code and sign in by using other methods --> Get a verification code and sign in with two-factor authentication - Apple Support
- Use a Trusted Device. If you have a trusted device (Mac, iPhone, iPad, Watch running the minimum system requirements; see System requirements for iCloud - Apple Support ), the verification code is displayed automatically on your trusted devices or manually using Settings.
- Get a code from an Account Recovery Contact --> Set up a recovery contact for your Apple Account - Apple Support
If you find you can’t sign in, reset your password, or receive verification codes, you can request account recovery (If you forgot your password and can’t sign in to your Apple Account - Apple Support ) to regain access to your account. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity. The post by shoeluvr13 at this link provides additional detail about the account recovery process: Activation Lockout Issues - Apple Community