A small chip and some memory are housed on the card. The most important feature of these modules is network authentication. For this, keys and algorithms are stored on the card, which encrypt both voice and signalling data, making it unreadable by unauthorised individuals. Additional functions include storing temporary, network-related data on the card, such as preferred or blocked networks. The phone user can store phone numbers, notes or text messages on the SIM card. When the SIM card is placed in another phone the user-specified address book is transferred with it.
SIM cards are available in various sizes. At the beginning of the mobile era SIM cards in the credit card format of 85.60 x 53.98 x 0.76 mm (LxWxH) were the market standard, but the increasing miniaturisation of devices has seen ever smaller formats being created. Today's mobile phones feature Mini-SIMs are (25 x 15 x 0.76 mm), Micro-SIMs (15 x 12 x 0.76 mm) and Nano-SIMs (12.30 x 8.80 x 0.67 mm).
Some mobile phones have dual SIM capability or can be operated with dual SIM adapters. This allows two SIM cards to be used simultaneously, so that one mobile device can have two different contact numbers. These are either active simultaneously or the mobile phone can switch between them. These mobile phones make it possible, for example, to combine, yet strictly separate, both work and personal communications on a single device