From : Two-factor authentication for Apple ID - Apple Support
To use two-factor authentication, you need at least one trusted phone number on file where you can receive verification codes. You can update your trusted phone numbers when you follow these steps:
- Go to your Apple ID account page.
- Sign in with your Apple ID.
- 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.