Dual SIM, or subscriber identity module cards, are inserted into a smartphone that accepts dual SIM cards, and a program in the phone allows the user to switch between SIM cards. The SIM cards can be used to perform separate functions simultaneously.
Dual SIM cards allow for a variety of optional configurations to save money, like being able to access different networks and having a number for business calls and a different number for personal calls. If calls to people on the same networks are free, which they usually are, an individual could have one SIM card to call certain people and another SIM card to call other people who are on a different network.
People who travel a lot can benefit financially by having a second SIM card that works on the foreign network they are visiting. Otherwise, they are charged expensive international roaming charges. Because the user has his home network SIM card, he is still able to receive calls from family and friends from home at the same time. Before dual SIM cards came about, it was necessary to remove one SIM to replace it with another, and the phone would not work on different networks unless the phone was unlocked.