A SIM card (subscriber identity module) is a small smart card that securely stores the identity of a mobile subscriber and the secret key used to authenticate a phone on a cellular network. Inserting a SIM links a device to a phone number and a carrier plan.
Why it matters
The SIM holds identifiers such as the IMSI (international mobile subscriber identity) and the ICCID, which is printed on the card. The secret key never leaves the chip; the network sends a challenge and the SIM calculates the response. Moving the SIM to another unlocked phone moves your number and plan.
Physical sizes shrank over time: mini, micro, and nano, with nano measuring about 12.3 by 8.8 mm. Many devices now also support eSIM, a chip soldered into the device and loaded with a carrier profile digitally.
Example
If your phone is stolen, calling your carrier to block the SIM stops calls and data on that card. A thief who swaps SIMs cannot use your number, but SIM-swap fraud is the reverse: an attacker persuades the carrier to move your number to their SIM, which is why SMS codes are a weak second factor.
Common mistakes
- Forgetting that a carrier-locked phone may reject other SIMs.
- Using SMS-based verification for high-value accounts when stronger methods are available.
- Cutting a SIM to fit without checking the chip layout.
Related
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