The third category is related business codes, such as personal identification code (PIN), unlock code (PUK), billing rate, etc. The fourth category is the telephone directory. Almost all user information is stored in the SIM card, so the SIM card is also called "user information identification card". SIM card can be divided into big card and small card. The small card is called "plug-in SIM card", which is the chip on the big card.

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.