What does Apple's credit number mean?

Trusted phone number refers to the number that can receive verification code through SMS or automatic voice phone.

You must verify at least one trusted phone number to register for two-factor authentication. You should also consider verifying another phone number that you can use, such as a home phone number or a number used by relatives and friends. If you are temporarily unable to use your primary number or your own equipment, you can use this alternate number.

It enhances the security of AppleID and all personal information stored through Apple.