into their existing handset. However, it is not always possible because some carriers (e.g., in U.S.) SIM-lock the phones that they sell, preventing rival carriers' cards from being used.

The use and content of the card can be protected by use of PIN codes. One code, PIN1, can be defined to control normal use of the phone. Another code, PIN2, can be set, to allow the use of special functions (like limiting outbound telephone calls to a list of numbers).