How do you add another phone number

1. Go to “Settings.”
2. Tap on your Apple ID.
3. Go to “Password & Security.”
4. Tap “Edit” to the right of “Trusted Phone Number.”
5. Tap “Add a trusted phone number.”
6. Select the country, enter the phone number, and select how you want to verify the phone number. Tap “Send.”
7. Enter the verification code received by the phone number you added.
8. Enter your device’s passcode.
9. The new phone number will be added to the “Trusted Phone Numbers” list.

See" Two-factor authentication for Apple ID - Apple Support

Read the sections about:

- Trusted phone numbers

- Manage your account. If you can still sign into your account, go to https://appleid.apple.com/ and add your new number, then sign out of iCloud and back in again on your devices.

- What if I can't access a trusted device or didn't receive a verification code?

And if necessary:

- If I can't sign in, how do I regain access to my account?