The use of roaming SIM cards in its most common form is in normal voice applications such as mobile phone calls. The common application of roaming SIMs for voice is where mobile calls are automatically routed to, and made on, the least cost network. This typically means that incoming calls are free, no matter which network a mobile user is on. This also means that a caller enjoys the lowest cost when making a call, significantly reducing call costs, especially compared to normal network charges for International Roaming. Global roaming SIMs are very often combined with
callback technology, whereby the user dials a number in the normal way, but the call is intercepted by an application on the SIM card and turned from an outbound call to an inbound call which the user answers. This ensures that the call travels exclusively through the least cost route, and also it is taking advantage of the fact that inbound call charges are typically lower than outbound ones. Some providers achieve this automatic call interception and callback by encoding a
program onto the SIM card. Other providers use Multi-IMSI (
International Mobile Subscriber Identity) technology to lower the cost of roaming. In this case, there is a program on the SIM card that selects the lowest cost IMSI (or 'profile') to use in a specific country. Increasingly, data services are being added to roaming SIM cards to reduce the cost of roaming data charges. Mobile users are increasingly using data services, and it can be very difficult to predict the cost of using data because it is invoiced based on volume. == Machine to machine ==