A SIM card is a phone card. You cannot receive messages without a SIM card.

1. The SIM card is the only customer identity in the field of mobile communications. It stores digital mobile phone customer information, encryption keys and other contents on the chip card, which can be used by the mobile communication network to identify the customer's identity and encrypt the customer's voice information during calls.

2. There are currently three types of SIM cards on the market: standard SIM cards, micro SIM cards, and nano SIM cards. The three models are ordered from large to small. Standard SIM cards have been abandoned by current smart phones with the development of the times, and are mostly used in elderly phones and feature phones. The latter two types of micro SIM cards and nano SIM cards are currently the mainstream of smartphones on the market.

Extended information:

The SIM card adopts a new microcontroller and memory management structure, so the processing function is greatly enhanced. The logical structure of its intelligent features is a tree structure. All characteristic parameter information is expressed in the form of data fields. There are three types of data information stored in the SIM card:

1. Information related to the cardholder and all business information that the SIM card is prepared to provide in the future. This type of data is stored in the root directory;

2. Information unique to GSM applications, this type of data is stored in the GSM directory;

3. Used by GSM applications Information, which can be shared with other telecommunications applications or services, is located in the telecommunications directory.

Baidu Encyclopedia-SIM Card