The below is based on the section "About trusted phone numbers and trusted devices" in the article Two-factor authentication for Apple Account - Apple Support
To see, add, or change your trusted phone numbers use one of these methods:
● On your iPhone, iPad, or iPod touch: Go to Settings > your name > Sign-In & Security. Tap Two-Factor Authentication, tap Edit (above the list of trusted phone numbers), then enter your device passcode when prompted. Add or remove the number, then tap Done.
● On a Mac computer choose Apple menu > System Settings (or System Preferences), then click your name (or Apple ID). Click Sign-In & Security, then add or remove a trusted phone number.
● Using a web browser:
1) Go to https://account.apple.com and sign in.
2) Follow Account Security options until you find one letting you change trusted numbers,
3) Click "+" to add a new number (you may be required to verify this number).
4) If you wish to remove an old number, click "-" next to the number.
Similar information for the iPhone can be found here --> Use two-factor authentication for your Apple Account on iPhone - Apple Support
● If an app or feature on your device is not working, not appearing, or is restricted (grayed out), a frequent reason is it may have been disabled using Screen Time, or by an IT administrator if you are using a device owned by an employer or educational institution.
- Read this support article If your Apple ID or other options are greyed out in Settings —> If your Apple Account is grayed out in Settings - Apple Support
Tap your Settings app > Screen Time > Content & Privacy > scroll to Allow Changes and turn on.
- If the device is managed (Settings > General > VPN & Device Management), contact the IT department that is doing the managing.