Make cheaper calls from your mobile using RebelSim Call Through

Pros:
Set it up and forget it approach for cheap international calls from mobile phones
Cheap
Will work with any phone that uses regular size SIM cards (the phone can be locked or unlocked, from anywhere in the world, on any carrier, etc)
Can be repurposed with 3 different firmwares ( one for call through, one for unlocking HTC phones, and the other a different approach for cheap mobile calls)
Simple to configure
Cons:
Latest firmware does not allow to use access numbers that require you to enter a PIN or ID number to use (though it can with firmware v2.1, which is to be released soon)
Can be a bit tricky to sandwich it with your existing SIM card and push into the SIM slot on some phones
Doesn’t work with micro SIM cards (won’t work with iPhone 4)

Info About Application Development for STK Sim Tool Kit

The STK Sim Application Toolkit is a Universal Standard used to create applications that reside on network simcards. Normally it is not possible to load applications on to a network simcards and only the network can supply functions that use the STK. Solutions Point Limited Specialise in Sim Overlay technology which consists of a FPCB [...]