I have never thought about this question of how does a SIM card work till I run into this problem that I ordered Bouygues sim card online but they failed to send me home and I finally got it from their physical store immediately after I went there. Before, I thought a SIM card connects to a specific phone number, but it seems not the case because the staff in the store gave a new card to me and I can use it immediately after having activated it in my phone. What I am confused about is why a random new card can recognize my phone number?

After doing some research in google, I found that a smartphone recognizing a SIM card works by recognizing a seriel number, called International Mobile Subscriber Identity (IMSI) in the card, and then the information of this number is sending by the phone to the network, in which the phone number related to the IMSI is stored in the database (I could have thought about it!).

A detailed procudure of communication between the smartphone and the cellular network is as follows:

Sometimes we got used to the things that we are using everyday, but i didn’t ask a lot why and how they work?