The present disclosure provides a method and a system for verifying identity of a user and a system for generating and maintaining verification codes. The method and the system use system-generated caller numbers as verification codes. When an application system needs to verify the identity of a user, the verification system generates a caller phone number and uses a communication switching system to place a call originating from the caller phone number to the user's phone. The user's phone has a caller ID display function to automatically display the caller number, which serves as a user identity verification code. The user provides the received caller number to the application system for user identity verification. No text messaging or other ways of communication is required. The method and the system can achieve reliable user identity verification. A centralized system may support multiple application systems for user identity verification.