Trusted phone number

See the sections for trusted phone number here:

Two-factor authentication for Apple ID - Apple Support

Manage your trusted phone numbers

To use two-factor authentication, you need at least one trusted phone number on file where you can receive verification codes. Consider verifying an additional trusted phone number other than your own phone number. If your iPhone is your only trusted device and it is missing or damaged, you will be unable to receive verification codes required to access your account.

Update your trusted phone number on your Apple ID account page

1. Go to appleid.apple.com.
2. Sign in with your Apple ID.
3. Go to the Security section and click Edit.

If you want to add a phone number, click Add a Trusted Phone Number and enter the phone number. Choose to verify the number with a text message or automated phone call, and click Continue. To remove a trusted phone number, click  next to the phone number you want to remove.

Update your trusted phone number through account recovery

1. Go to appleid.apple.com.
2. Sign in with your Apple ID.
3. Select “Didn’t get a verification code?”, then select More Options.
4. You'll be redirected to iforgot.apple.com. 
5. Complete your account recovery request. Learn more about account recovery. 

Good luck!