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.