|This article does not cite any references or sources. (May 2010)|
Roaming SIMs can be defined as mobile phone SIM cards that operate on more than one network within their home country. They currently have two main applications, least cost call routing for roaming mobile calls and machine to machine.
Using a normal network locked SIM, travellers for instance, can use their own mobile phone (if it's enabled for roaming), in any country that has a roaming agreement with their home network, or for global networks like Vodafone, with another Vodafone OpCo. This manifests itself to most users when they receive a text message welcoming the traveller to a local network. However, 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 which ever 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 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. United World Telecom or Telecom North America are two of the providers in the US using this technology.
Some providers achieve this automatic call interception and callback by encoding a program onto the Sim card. For example Smart Voyagerin UK,UWTMobile, and MySims2Go utilise this method to forward the call and trigger the callback.
Increasingly, data services are being added to roaming SIM cards to reduce the cost of roaming data charges. Mobile users are increaslingly 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, for example in vehicle tracking systems, amongst other things, where complete network coverage is required but can only be achieved by using all the available networks. A roaming SIM can switch to any network, ensuring the best coverage. The cheap calls associated with Roaming SIMS are also paramount to enable certain machine to machine applications to be economically viable. It is anticipated that our exposure to Roaming SIMS, applied in a range of scenarios, will become prevalent as more and more applications for them are brought to market.
For some applications (particularly where regular travel between two countries is the main purpose) a Dual SIM 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.
There are plenty VOIP applications that you can install on your Smart Phone and call international numbers; rates are much lower than those of Roaming Sims. The downside of using VOIP for international calls is that you will have to find a Wi-Fi spot and in some cases voice quality is low.