What does mobile phone two-way authentication 999 mean?

The user of 12306 sends a short message "999" to 12306 using the mobile phone number left when registering. Mobile phone two-way authentication 999 is the official of 12306. It is the mobile phone number left by the user when using 12306. It sends a short message "999" to 12306 to prevent identity information from being registered. China Railway Customer Service Center (12306) is an important window for railway customers, which integrates passenger and freight transportation information of the whole railway and provides passenger and freight transportation business and public information inquiry service for social and railway customers.