A subscriber identity module is a removable smart card for mobile phones. SIM cards store the required information to identify the mobile device. It also contains data required for voice encryption to make listening in on calls almost impossible (except when the wireless carrier itself is doing the eavesdropping).

In this way the customer ID (and personal number) is tied to the SIM card and not to a certain mobile phone. This allows for a seamless interchange of the same SIM card between different GSM mobile phones.

SIM cards also serve as storage for SMS messages and the user’s contacts. Current SIM cards can store up to 250 name/number pairs and up to 50 SMS text messages.

The SIM card cannot store multiple numbers per contact or other more complex information. This means that if you copy your contacts info from the phone memory to the SIM memory, contacts get broken up into as many entries as there are numbers for each individual contact and discards the other information.

All GSM phones and most iDEN phones require a SIM card to operate.

There are certain types of phones (CDMA, TDMA, AMPS) that do not use a SIM. Instead, the required data is programmed directly into the phone.

Replaceable SIM cards come in four standard sizes:

• Full-size (85.6mm × 53.98mm × 0.76 mm)
• Mini-SIM (25mm x 15mm x 0.76mm)
• Micro-SIM (15mm x 12mm x 0.76mm)
• Nano-SIM (12.3mm × 8.8mm × 0.67mm)

The eSIM or Embedded SIM comes in one size:

• eSIM (6mm x 5mm x <1mm) Non Removable

Dual-SIM

This specifies whether a device is capable of supporting two SIM cards. The two major types of dual-SIM phones are active and standby. Dual-SIM Standby (DSS) requires the user to specify which of the two SIMs is able to make and receive calls, while Dual-SIM Active (DSA) enables both cards to receive calls at the same time. This latter feature usually requires an additional transceiver for the secondary SIM card, and as such consumes more battery life. More recent models feature Dual SIM Dual Standby (DSDS) technology which enables them to have two active SIMs with only one transceiver