The SIM card and the switch use the authentication algorithm to calculate the authentication key and 8-digit random number at the same time. If the calculation results are the same, the SIM card will be recognized, otherwise the SIM card will be rejected and the user will not be able to make a call. The SIM card can also use encryption algorithm to encrypt the conversation voice to prevent eavesdropping.

SIM card is called "user identification card". It is actually a smart card containing large-scale integrated circuits, which is used to register important data and information of users. The most important function of SIM card is authentication and encryption. When a user moves to a new area to make or receive a call, the switch must authenticate the user to determine whether it is a legal user.