What is the *31# in front of the mobile phone?

calling number hiding-in normal communication, a mobile user with the function of "calling number hiding" dials the other party's mobile phone or fixed phone with caller ID function, and only the corresponding prompt is displayed on the other party's mobile phone or fixed phone, but the calling user's mobile phone number is not displayed. (2) Two functions, free choice: 1. Optional caller hiding: Usage: The normal state is to display the number. If you need to hide your mobile phone number when making a call, please dial "#31#" before the called number. 2. Completely hidden calling: (3) Usage: The normal state is hidden number. If you need to display your mobile phone number when making a call, please dial "*31#" before the called number. (4) Clients: Hainan Mobile Global Connect and Hainan Connect customers. (5) Charging standard: 3 yuan/month, which is charged on a daily basis. (VI) Special reminder: 1. After the calling number hiding service is opened, calling mobile phones in the province can be used normally, calling fixed phones or roaming out of the province, and whether it can be used is related to the setting of the peer system. 2. When a customer applies for "optional caller hiding" or "complete caller hiding", he can only choose one of them.