The necessary data for a mobile device’s identification is stored on SIM cards. Additionally, it has the information needed for voice encryption, which makes listening in on conversations nearly impossible (except when the wireless carrier itself is doing the eavesdropping).

Thus, the user ID (and personal number) is connected to the SIM card rather than a specific mobile device. As a result, the same SIM card can be easily switched between various GSM cell phones.

SMS messages and the user’s friends can also be stored on SIM cards. Currently, SIM cards can hold up to 50 SMS text messages and 250 name/number combinations.

Multiple phone numbers per contact or other more complicated data cannot be stored on the SIM card. This means that when you transfer your contacts’ information from the phone’s memory to the SIM memory, the remaining information is discarded and contacts are divided into as many entries as there are numbers for each individual contact.

The majority of iDEN phones and all GSM phones need a SIM card to function.

A SIM card is not required for some phone models (CDMA, TDMA, AMPS). Instead, the necessary information is built right into the phone.

Four common sizes are available for replacement SIM cards:

  • 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

The picture above demonstrates how the SIM card’s capacity has shrunk over time.

The full-size or 1FF (1st Form Factor), which measures 85.60 mm 53.98 mm 0.76 mm, was the first to be produced. A mini-SIM or 2FF (second form factor), which has the same thickness but measures 25 mm long by 15 mm wide and has one of its corners cut to avoid misinsertion, was placed after it. The third form factor, or micro-SIM, was next, with measurements of 15 mm 12 mm.

The 12.3 8.8 0.67 mm nano-SIM, also known as the 4FF (4th Form Factor), was launched in 2012. Nano-SIM cards can be made compatible with devices that have Micro-SIM and Mini-SIM ports by using adapters. With the use of an adaptor, a Micro-SIM card can also be installed in a Mini-SIM slot.

The embedded universal integrated circuit card (eUICC), also known as the eSIM or embedded SIM, was launched in 2016. It performs the same functions as a detachable SIM and is manufactured by being soldered onto the motherboard of a device.

A SIM card has two numerical codes attached to it. The first is the Personal Identity Number (PIN) that must be entered each time the user turns on the device (this can be turned off via the phone settings).

The user has only three tries to enter the PIN number. If any one of the three is off, the card is secured and must be unlocked using a PUK (Personal Unblocking Key). Prior to the card being permanently locked and rendered useless, only ten attempts at entering the PUK are allowed.