{{Unreferenced, date=May 2010 A roaming SIM is a mobile phone

SIM card A typical SIM card (mini-SIM with micro-SIM cutout) A file:Simkarte NFC SecureElement.jpg">thumb"> thumb">A TracFone Wireless SIM card has no distinctive carrier markings and is only marked as a "SIM card" A SIM card (full form Subscriber Ide ...

that operates on more than one network within its home country. Roaming SIMs currently have two main applications, least cost call routing for roaming mobile calls and machine to machine. Using a normal network locked SIM, travelers can use their own roaming enabled mobile phone in any country that has a roaming agreement with their home network, or for global networks like

Vodafone Vodafone Group Public limited company, plc () is a British Multinational corporation, multinational Telephone company, telecommunications company. Its registered office and Headquarters, global headquarters are in Newbury, Berkshire, England. It ...

, with another Vodafone OpCo. This manifests itself to most users when they receive a text message welcoming the traveler to a local network. Once they return home their SIM will only work on the network with which they have a contract. A roaming SIM however, also known as a global roaming SIM, will work with whichever network it can detect, at home or abroad.

Roaming mobile calls

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 a 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 Callback may refer to: * Callback (comedy), a joke which refers to one previously told * Callback (computer programming), executable code that is passed as a parameter to other code * Callback (telecommunications), the telecommunications event tha ...

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 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

This technology is also used in a number of machine to machine applications where devices communicate with one another, such as vehicle tracking systems. By switching between multiple networks, more complete network coverage is obtained and costs are minimised through least-cost routing.

Alternatives

For some applications (particularly where regular travel between two countries is the main purpose) a

Dual SIM Some mobile phones support use of two SIM cards, described as dual SIM operation. When a second SIM card is installed, the phone either allows users to switch between two separate mobile network services manually, has hardware support for keeping ...

can be considered as an alternative. They have the advantage that it is possible to buy a local SIM card and use that next to the primary SIM card.

Voice over IP Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Interne ...

apps (

softphone A softphone is a software program for making telephone calls over the Internet using a general purpose computer rather than dedicated hardware. The softphone can be installed on a piece of equipment such as a desktop, mobile device, or other compu ...

s) may be installed on

smartphone A smartphone is a portable computer device that combines mobile telephone and computing functions into one unit. They are distinguished from feature phones by their stronger hardware capabilities and extensive mobile operating systems, whic ...

s to inexpensively call international numbers. As these use

Wi-Fi Wi-Fi () is a family of wireless network protocols, based on the IEEE 802.11 family of standards, which are commonly used for local area networking of devices and Internet access, allowing nearby digital devices to exchange data by radio w ...

where available, costs may be substantially lower. Voice quality using VoIP for international calls may vary. Mobile telecommunications