There are three operating voltages for SIM cards: 5 V, 3 V and 1.8 V (ISO/IEC 7816-3 classes A, B and C, respectively). The operating voltage of the majority of SIM cards launched before 1998 was 5 V. SIM cards produced subsequently are compatible with 3 V and 5 V or with 1.8 V and 3 V.
Dual SIM phones are now made by some mobile phone manufacturers, which save the user from carrying around a separate phone for every number. There are two types: the first allows one to switch between the SIMs, whilst the second allows both SIMs to be active simultaneously.
SIM operating systems come in two main types: native and Java Card. Native SIMs are based on proprietary, vendor-specific software, whereas the Java Card SIMs are based on standards, particularly Java Card, which is a subset of the Java programming language specifically targeted at embedded devices. Java Card allows the SIM to contain programs that are hardware independent and interoperable.