Authentication based on telephone number recycling

A method and system for authenticating a user is provided. In some embodiments, a security system determines whether the time since the last authentication was successful is less than a recycle telephone number period (e.g., the minimum time before which a telephone number might be assigned to a new user). If the time is less than the recycle telephone number period, the security system performs a primary authentication of the user based on a telephone number received from the user. When the primary authentication is successful, the security system indicates that the user has been authenticated. When the time is not less than a recycle telephone number period, the security system performs a secondary authentication of the user. When the secondary authentication is successful, the security system indicates that the user has been authenticated.