A subscriber identity module (SIM) is a removable smart card for phones. SIM cards store the required data to recognize the cell phone. Also, It contains information required for voice encryption to make listening in on calls almost impossible.
Also the client ID (and individual number) is attached to the SIM card and not to a specific mobile phone. This allows a consistent exchange of the same SIM card between various GSM phones.
SIM cards additionally serve as storage for SMS messages and the client’s contacts. Current SIM cards can save up to 250 name/number and up to 50 SMS instant messages.
The SIM card can’t store different numbers per contact or other increasingly complex data. This implies that if you copy your contacts data from the telephone memory to the SIM memory, contacts get separated into many entries as there are numbers for every individual contact and removes of the other information.
All GSM telephones and most iDEN telephones require a SIM card to work.
There are kinds of telephones (CDMA, TDMA, AMPS) that don’t use a SIM. Rather, they required information is programmed straight into the mobile phone.
Replaceable SIM cards come in four standard sizes:
Full-size (85.6mm × 53.98mm × 0.76 mm)
Mini (25mm x 15mm x 0.76mm)
Micro SIM (15mm x 12mm x 0.76mm)
eSIM (6mm x 5mm x <1mm) Non Removable
Nano-SIM (12.3mm × 8.8mm × 0.67mm)