Your Service Provider supplies you with a Subscriber Identification Module (SIM) Card or a Universal Mobile Telecommunications System Subscriber SIM (USIM) Card. A SIM or USIM Card is a card commonly used in a GSM phone. The card holds a microchip that stores information and encrypts voice and data transmissions, making it extremely difficult to listen in on calls. The USIM Card also stores data that identifies the caller to the Distribution Partner.