Subscriber Identity Module (SIM), commonly referred to as a SIM card, functions as a small chip that carries crucial details about a mobile phone user's network service.

Fundamentally, SIM card works as a key to accessing a cellular network, allowing mobile phone owners to send and receive calls. In addition, it also aids in keeping important phone numbers, text messages, and even in some cases network-specific data.

There are generally three sorts of SIM cards present in the market: standard, micro size, and nano. These types differ in physical dimensions, but produce similar functionality.

Each SIM card is identified by a unique IMSI number, which is consists of a fixed length numeric code. This distincts each subscriber within the network.

Moreover, every SIM card is programmed with a specific Authentication Key (Ki), which acts for guaranteeing data exchange over the network.

Talking about these SIM cards' functions, they hold details such as a user's phone number, service information, and security data. Apart from these, they allow additional services including call advancing, call waiting, and voicemail.

In closing, we see that SIM cards are crucial parts of our mobile phones. They are like the identity cards for our phones, making them function efficiently. Knowing your SIM information helps in solving issues related to your network service.

Moreover, being knowledgeable about your SIM card more info also can aid in making decisions regarding upgrading your phone, switching service plans, and even traveling internationally.