Delete my phone number from iCloud security

The Apple ID is already signed in an iPhone , and at least one trusted number is verified in the account to set up the account successfully , 14 days have been crossed the 2FA account is permanent , you can't add zero trusted number in the account , even if you had tried to add no trusted number when you were creating the account it won't set up .

A trusted number is essential , see the articles Two-factor authentication for Apple ID - Apple Support

Get a verification code and sign in with two-factor authentication - Apple Support

See the article Manage two-factor authentication from iPhone - Apple Support

Add or remove a trusted phone number

When you enrolled in two-factor authentication, you had to verify one trusted phone number. You should also consider adding other phone numbers you can access, such as a home phone, or a number used by a family member or close friend.

1. Go to Settings  > [your name] > Password & Security.
2. Tap Edit (above the list of trusted phone numbers), then do one of the following:

• • Add a number: Tap Add a Trusted Phone Number.
  • Remove a number: Tap next to the phone number.

Trusted phone numbers don’t automatically receive verification codes. If you can’t access any trusted devices when setting up a new device for two-factor authentication, tap “Didn’t get a verification code?” on the new device, then choose one of your trusted phone numbers to receive the verification code.