trusted number

Go to http://appleid.apple.com, from : Two-factor authentication for Apple ID - Apple Support

You can update your trusted phone numbers when you follow these steps:

1. Go to your Apple ID account page.
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.